British fighting formations vs. French. The books describe the French attacking in deep columns to act as battering rams on the enemy lines whereas the British used long thin lines that allowed many more muskets to be used in a firing line. It seems whenever a British army faced a French army using these tactics, the British won.

The French battalions deployed in Spain were not the best quality: by the time the Peninsular War started, Napoleon was starting to scrape the bottom of the barrel for manpower, and he routinely combed out the best troops from Spain for the wars against Austria and the invasion of Russia. Also, Napoleon only came to the Peninsula in person once: his most recent biographer, Andrew Roberts, suggests that he took one look at the terrain of Spain and realised that it wasn't going to be the kind of rapid, decisive war of manoeuvre that he excelled at, but a long, grinding, attritional campaign that would diminish his standing and reputation, if it could be won at all. Consequently, the best troops of La Grande Armée never fought in the Peninsula.

Consequently, French commanders in Spain had to adapt their tactics (which otherwise comprised the best tactical system in Europe, probably until 1814). Much like the levée en masse of the early French Revolutionary Wars, under-trained, under-motivated troops had to be marched in massed formations, the column. The Grande Armée did use the column as a manoeuvre formation, but with the expectation that it would swing into line before engaging the enemy, attack with musket fire, and then close with the bayonet, supported throughout by a cloud of skirmishers. During the attack through the Austro-Russian centre at Austerlitz, the leading two regiments of Marshal Soult's corps moved from column into skirmish order at a run and engaged the enemy, before another two regiments in column supported by nine heavy cannon drove the enemy from the field. While the French retained an excellent artillery arm in the Peninsula, their troops generally lacked the skill to execute such manoeuvres.

Then there's the geography, which as I've mentioned, Napoleon probably noticed early on. Spain and Portugal generally consist of steep, heavily-cultivated hillsides that were nightmarish for trying to execute attacks in close-order formations. Add to that Wellington's superb eye for selecting defensive terrain. The British routinely deployed on the reverse slope of hills, protected from French artillery and denying the French the opportunity to spot their positions. Consequently, the French would have to make an exhausting climb up a steep hill, with plenty of bushes, terraces and treelines to disorganise them, and under fire from British skirmishers on the crest (equipped with the excellent Baker rifle, which allowed them to pick off officers, NCOs and drummers, thus adding to the confusion). When they finally crested the hill, the British would be deployed in line mere yards away, at which point it was far too late to deploy into line and the only option was to batter through.

To summarise, the French were routinely beaten in battle in Spain not because their tactical system was poor, it was because they deployed second-rate troops that could not execute complex manoeuvres, and were fighting in terrain that precluded such deployments anyway. There was also a sense that, having fought in such a way during the French Revolutionary Wars, attacking in column was doctrinally the "right" thing to do with such troops and in such terrain.

Sources:

Paddy Griffith, French Napoleonic Infantry Tactics, 1792-1815

Philip Haythornthwaite, British Napoleonic Infantry Tactics, 1792-1815

Richard Holmes, Redcoat: The British Soldier in the Age of Horse and Musket

Andrew Roberts, Napoleon the Great

Peter Snow, To War with Wellington

Okay this is definitely a topic I’ve been hoping would pop up here for a while now, and I wanted to base my response in two parts, building slightly off of u/Rob-With-One-B’s response and talking about the Sharpe series as literature.

First off, he is absolutely right that French troops in the Peninsula, especially after ~1811 and the failure of Massena’s invasion of Portugal, lost a lot of the edge that can be seen in French troops outside of Spain because they slowly became dominated by recent recruits and conscripts. These troops, as he mentioned, could not complete the type of complex maneuvers that Napoleonic warfare required, such as advancing in even-line and maintaining fire discipline. However, while I don’t disagree with him on facts, I think the military history of the French in the Peninsula tends to be conflated with British perspectives that diminish their abilities more than they actually were.

I’ll be taking a bit from a slightly different question on the Peninsular war I answered here

Before I tackle the points that Bernard Cornwell underlines, I’ll begin by trying to contextualize how he describes the fighting in Spain. In his series (though keep in mind it’s been years since I’ve read them), the British advance and victory against the French seems inevitable. The British kept a well-equipped, well-led, and sizeable regular army in the theatre that forced the French to consolidate armies to defeat it, while the Spanish after the disaster of 1808-1810 focused on encouraging and supporting guerrilla. The fact that a British regular army existed in the Peninsula had a major impact in how the French carried out the occupation, forcing them to hedge between unifying and splitting forces as you and u/Rob-With-One-B touch on, and Bernard Cornwell does a fantastic job of painting what this British army looked like and how it fought. Viscount Wellington (he wouldn't be granted the Dukedom until after Waterloo) consistently and decisively led a "regular" campaign against the French while Spanish regular forces perpetually failed to recreate the victory at Bailén. However, while the British regular army did win consistent victories against the French troops, Cornwell tends to gloss over the fact that despite winning local battles, the Marshals’ maneuvers and the fear of being encircled by much larger numbers forced Wellington to retreat back to the safety of his base in Lisbon multiple times (in 1809 after the battle of Talavera, 1811 after Ciudad Rodrigo, and 1812 after Burgos). All this is to say that the war against the French was not as easy as Cornwell tends to (if I’m remembering correctly) paint it, for example, Marshal Suchet (the only general to earn a baton in Spain) was never defeated by a British army, and he and Marshal Macdonald held a suprisingly firm control of the provinces of Aragon and Catalonia, while Marshal Soult “pacified” Andalusía through sheer repression to the point that guerrilleros really weren’t a problem there (”...that iron hand...having so quited the countryside that single French soldiers could travel safely between Seville and their lines outside Cádiz”). Quite honestly before 1812 and Napoleon’s doomed Russia campaign, it was by no means assured that the Allies would win in the Peninsula, Wellington’s force made up almost the totality of all regular troops Britain could afford to use to fight on the continent, if the French ever did succeed in surrounding him and destroying his expeditionary force, like Napoleon did in 1808, the French could have devoted themselves completely to counter-insurgency campaigning, which John Elting shows was not as hopeless as the British (and Spanish for that matter) tend to remember it. I can (and gladly will!) go into much more detail how the Spanish guerrilla wasn’t exactly how it has been remembered by the British and Spanish narratives, but suffice it here to say (this is dragging on) that absolutely the French were forced to spread out to put them down, but it was not an unwinnable fight by any means.

To tackle the three points you bring up in turn:

Not only were the French simply used to “living off the land” (as were most armies at the time, Britain was an exception, not a rule), this was a necessity in Spain because logistically, keeping an Army supplied with only your own food wasn’t feasible at the time, it required huge stores of food at predetermined depots and this slowed advances to a crawl (”English officers agreed that the French could outmarch them and find food enough in an apparently barren countryside where Englishmen would go hungry if their supply trains could not keep up.”) This is partly why Wellington was forced to retreat despite his victories, he could not advance anywhere near as quickly as the French who, even as conscripts, made war by marching as fast and far as possible to suprise enemy columns. Combine this with the fact that most Spaniards detested the French anyway, no matter if they bought their food or requisitioned it, and foraging was a necessity, not a choice for French forces.

This is a point that Cornwell brings up again and again in his novels, but (once again for space’s sake) he overstates how superior the English line was to the French column. In fact, the French simply marched in columns, they fired in lines three-ranks deep just like most everyone else (the British fired in lines two-ranks deep). The reason the British troops were more effective than the French in Spain has far less to do with superior British understandings of the line, than ineffectual French generalship in Spain, conscripts who could not pull of complex maneuvers such as the switch from column to line in a firefight, and the absolutely true fact that British soldiers were unparalleled in their weapon-handling ability, they could fire fast and accurately in a time were both were tough to achieve. The Grande Armée’s success came from it’s ability to exploit gaps in enemy lines and breach them in an instant (i.e, from positioning) more than their ability to out-shoot the enemy. Also Wellington’s masterful command must be credited, this rant isn’t intended to diminish his genius, but rather contextualize it as it tends to be mythologized.

It’s absolutely true that the British effectively denied the French Empire any meaningful control of the sea after 1805, but the blockade wasnt as omnipresent as one tends to think. The Royal Navy did have a finite amount of ships to blockade thousands of miles of coastline.

To conclude, Cornwell does an absolutely fantastic job of creating a war-novel series that quite accurately portrays the British army of the time and how they operated. It is beyond clear he does his homework and his research shows in his writing. However, being an author not a historian he is still creating literature, not history. Thus he tends to leave out important historical contexts when describing the French. It’s not a horrible portrayal, but it tends to make the war against them seem easier than it really was, and he tends to buy into the romanticization of the guerrilleros (who were truly anything but) and that’s the aspect in which I’d say he’s most misleading in the Sharpe series. However it’s still a fantastic series!

u/Rob-With-One-B provided a great list of further reading, I’d just like to add the work from what I quoted, which is indispensable if you want to understand the Napoleonic French army: John R. Elting, Swords Around a Throne: Napoleon’s Grande Armée, 1988