So this is a relatively straightforward answer.

And that is, that a few of the units had seen recent combat, along with their senior leaders, but by no means was the entire force made up of veterans, much less ones who had fought in the major battles of the Peninsular War.

​

The core of Ross's expedition was 4 battalions of line infantry from the 4th, 21st, 44th, and 85th regiments, along with several Royal Marine formations, and some Royal Artillery batteries.

​

Ross himself was a career soldier, having commanded the 20th Regiment in battle several times before rising to higher command. Notably, he led them at Corunna in 1809, and after several years at home, he took them back to Spain in 1813, seeing action at Vittoria and Wellington's drive to the Pyrennes. In early 1814 he was wounded while commanding a Brigade at Orthez, and after several months of recuperating he was sent to America as senior Army officer for the coming campaigns proposed by Admirals Cochrane and Cockburn. Ross can very much be described as an experienced officer, if not the top notch, he was still very competent and used to leading large units on campaign in hostile territory.

​

But what about the units with him? Lets start with the least experienced, the 1/21. The regiment had spent most of the previous decade in the Mediterranean, helping in numerous small landing operations in Sicily, Italy, and Spain, or securing Britain's position in Egypt. It may not have been at the larger battles in Spain, but was certainly used to the sort of operation Ross was taking them on in the Chesapeake.

Next we have the 85th, a unit that had been converted about a decade before to Light Infantry, meaning they were expected to be competent in both the standard battalion role, but also able to parcel themselves out into companies or platoons for skirmishing, screening friendly forces, or forcing back enemy light infantry. Every Line Battalion had a Light Company for the job, but several entire units were also given the role. This often also made them effective for detached service, or expeditionary warfare, where the combat could be small ins scale but still very sharp. The 85th had been with Wellington for several years, from defending Portugal in 1811 at Fuentes de Onoro to the breakout into Southern France in late 1813.

​

Then comes the 4th, this unit fought damn near the entire duration of the British involvement in Spain, from Corunna, to 3rd Badajoz, Salamanca, Vittoria, ending at the river Nive in Dec 1813. It was earmarked to bolster the American theater along with the 85th and was withdrawn in Spring 1814 to be sent across the Atlantic.

And finally the 44th. It was semi-unique in that it had 2 battalions active on campaign at the same time. Usually 1 would be at home in depot, recruiting new men, either ahead of its own deployment or to be forward to the active part of the Regiment. But by 1813 the 1/44 was sent to Spain to join the 2/44 which had been there for several years already, it was withdrawn a few months later though and would also be sent to America. Another unit which did not perhaps have the longest list of battles fought in, but which had been seasoned by a year in Spain and France if nothing else.

The Royal Marine units supporting him, amounting basically to 2 battalions worth had spent most of the past 18 months either supporting actions in Spain, or North America. And while not necessarily the most cohesive units, it was part of the job description for detachments to be thrown together, reorganized on the fly, or renumbered as needed.

So we can see that taken as a whole, Ross had in essence a small division which could draw on a core of men who had seen combat, been acclimated to the rigors of active campaigning, and which was used to the sort of expeditionary warfare they were attempting. If not the crack units from Wellington's forces, they represented a reasonable enough snapshot of the British Army in 1814, with units both experienced from Spain, and in the smaller secondary theaters of the Napoleonic Wars.

You may find some answers in this post by u/enygma9753 about the involvement of British veterans in the DC campaigns.

Napoleon had abdicated in 1814. This meant that Britain could free up considerable resources in several punitive campaigns against America, including the burning of the U.S. capital. The Americans had attacked and burned the provincial capital at York (Toronto) in 1813 and the DC attack was regarded as part of the tit-for-tat burnings of enemy towns and forts that occurred regularly in the War of 1812.

Napoleonic veterans were also present in many of the 1814 campaigns on the Niagara frontier. One battle, Lundy's Lane, pitted British regulars and U.S. regulars in a vicious close-quarters fight in pitch-black conditions. British troops, who had seen the wars in Europe, found this battle esp. to be horrific for its savagery.

Although Napoleon's abdication seemed to mark the end of the European wars, the situation was still uncertain and Britain wanted to end the war with America quickly, as it was an inconvenient distraction. While Britain's aims in 1812-1813 were mainly the defence of their Canadian colonies, the 1814 campaigns were driven essentially by Britain's desire to force the U.S. to the negotiating table.

Britain was weary of nearly a quarter-century of war in Europe and Wellington urged British negotiators to settle the peace soon, aware that war in Europe could still reignite.

America was bankrupt and with their capital in flames, Maine in British possession and the New England states talking of secession, their negotiators stalled for time in order to prevent Britain from claiming territory they had captured in the war.

Both sides eventually came to terms in the Treaty of Ghent, which restored the status quo that had existed before the war.