The Royal Navy was far from surprised by the use of ironclads in the American Civil War. The RN had developed the Aetna-class of ironclad floating batteries for use in the Crimean War, though they saw no service (their French equivalents did see combat). The RN's first sea-going ironclad, the Warrior, was commissioned in 1861. She was a powerful ship, especially when compared to the coastal ironclads of the American Civil War. In essence, she was a traditional steam frigate, but with an iron hull and iron armour. She had forty guns and 4.5in of high-quality armour. Built with a high freeboard and masts to supplement her steam engines on long journeys, she was capable of crossing oceans without worry, unlike USS Monitor or CSS Virginia. By the end of 1862, the RN would have constructed or converted 9 ironclads, all on a similar scale to Warrior. Just four years later, that number had doubled.

The RN was, however, paying attention to the naval aspects of the American Civil War. While its ships were generally superior to the American vessels, there were still useful lessons that could be learned. Hampton Roads showed the difficulty of sinking ironclads with traditional weapons, as Monitor and Virginia spent the battle fruitlessly bombarding each other. To this end, the RN spent a considerable amount of time trying to improve the capabilities of its guns. A major step forward came in 1863, when General Sir William Palliser invented the chilled iron shot, which was soon named after him. This was an elongated, somewhat conical, projectile made of cast iron, cast in a chilled mould to harden the outside. They were good against wrought iron armour, but tended to break up when fired at steel. They could contain a black powder bursting charge. However, this often exploded when piercing armour, so was left out when expecting to face an armoured target. From 1863-1865, Woolwich Arsenal developed a series of rifled muzzle loaders (RML). These guns had six to nine rifling grooves, which would engage with bronze studs on the projectile. Firing Palliser shot, they could penetrate up to their calibre in wrought-iron armour at 1000 yards. They were effective but cheap guns, though the rifling system caused considerable erosion of the bore. Hampton Roads also suggested the value of ramming; Virginia's ramming of Cumberland caused the latter to sink more quickly than any gun engagement could. A number of steam rams also operated with some success during the riverine campaigns of the Civil War. These views were reinforced by Battle of Lissa in the Austro-Prussian War. In this battle, eight ramming attempts culminated in the sinking of the Italian Re d'Italia by the Austrian Erzherzog Ferdinand Max. Combined, this enshrined ramming as a key tactical option for much of the late 19th Century - even the Canopus class of 1896-8 were given rams.

Another key lesson for the RN was the perceived vulnerability of its bases to attack by ironclads (and the equal vulnerability of it's enemy's). The riverine and littoral campaigns of the American Civil War featured a number of engagements between Confederate forts and Union gunboats and ships, with what appeared to be frequent successes for the ships. The use of ironclads meant that forts could not effectively destroy the attacking force. Meanwhile, masonry forts proved to be poorly protected against shellfire, especially from rifled guns. These could also allow wooden ships to engage forts from outside the range of the fort's guns. In many cases, forts were successfully suppressed by shore bombardment, allowing for troops to be landed, or ships to be moved into the area the forts were protected. This meant that the systems of forts defending British naval bases like Portsmouth and Plymouth were obsolete, but the same was true for French bases. These facts encouraged the RN to build more monitors, which were seen as being the best choice for destroying forts. Ships like Glatton (laid down 1871) were constructed to destroy the forts of French ports like Brest or Cherbourg. Monitors were also seen as being far more effective at defending the British coastline than fixed fortifications, with many of these ships being described as 'coast defence ships'. Finally, those fortifications that were constructed went through a change in design. Rather than being made from masonry or brick as the old forts had been, there was a much greater focus on earthworks; though most of these forts were constructed inland, to protect forts such as Plymouth and Portsmouth from attack by French troops landed elsewhere, the principles were still the same. The sea forts of the Solent and Spithead, that had been commissioned by the 1859 Royal Commission, were completed as iron forts rather than stone as originally designed. There was also a move towards mounting fewer, larger guns which would be more effective against ironclad ships.

As well as inspiring 'coast defence ships' like Glatton, the monitor would also lay the seeds for the RN's battleships in the late 19th Century. Monitor and her successors were impressive ships, with considerable resistance to enemy fire and an ability to deal heavy damage to a target. However, it was unsuitable for the RN's requirements. Monitor had a lack of ventilation belowdecks and was a poor seaboat. It could not cross oceans or travel far from land, both thing the RN would need given its worldwide commitments. As such, the RN built no ships to the classic Monitor design. Instead, the RN's chief designer, Edward Reed, solved the flaws of the Monitor design by adding an armoured breastwork amidships, with turrets, ventilators and other openings positioned on it, with the first such ship being the Cerberus of 1867. This created a monitor capable of operating at sea, limited only by coal supplies. The RN's battleships of the late 19th Century would follow in Cerberus's footsteps. The breastwork would be extended to the edge of the hull, and then forward to its ends. The turrets would be replaced with lighter barbettes (rotating gun platforms with a ring of armour surrounding them), allowing for the freeboard to be raised. When the barbettes were given an armoured covering or gunhouse, the classic 'Pre-Dreadnought' was formed.

The most important naval lesson for the RN from the American Civil War, though, did not involve ironclads. Instead, it focused on commerce raiding and cruiser warfare, which used mainly unarmoured ships. The Royal Navy, in the years running up to the Civil War, had neglected to build cruisers (here meaning the general category that includes frigates, sloops and corvettes), favouring ironclads like Warrior. Similarly, commerce raiding and trade protection had been neglected in RN tactical thought. However, during the war, both the Union and Confederate navies carried out significant cruiser warfare campaigns. The USN carried a significant, and ultimately successful, blockade of the Confederate coast, capturing 1149 blockade runners and sinking another 335. The Confederates, meanwhile, used privateers and cruisers to harass Union trade. The Confederate cruisers captured or sank 209 ships, while their privateers captured another 52. While this was small compared to the number of ships in the Union merchant marine, it had an outsize effect on it, as ships were laid up, or transferred to other flags - just under half a million tons of Union shipping was transferred to the British flag (this represented about a fifth of the Union's sea-going merchant fleet in 1860). Several key lessons could be learned from these campaigns. The Union campaign showed the need for large amounts of cruisers to maintain an effective blockade. The Confederate campaign showed that raiders could operate world-wide, that raiders could not be effectively starved of coal (as they could take it from prizes or buy from neutrals), and that neutral nations might not be willing to properly enforce the rules of neutrality. The campaign also showed the difficulty of finding and capturing raiders, with the USN finding it difficult to locate their Confederate prey.

The RN's response to these successes was to start construction on a number of new classes of cruiser. They attributed Confederate success to a lack of fast Union cruisers. The Amazon, Blanche and Druid classes of corvette were built to outrun the fastest raiders available. In 1866, the USN commissioned the Wampanoag class of fast frigates. They had been ordered in 1863 "to sweep the ocean and chase and hunt down the vessels of an enemy", a description that could apply equally to British merchants as it did to Confederate cruisers. Theoretically capable of 16 knots and well armed (though in fact, they could not sustain the speed for long and could not mount a chase gun, a necessity for commerce raiding), they were a significant threat. In return, the RN built three frigates of the Inconstant class. These were generally excellent ships for their designed role, fast, seaworthy and long-ranged, while carrying a heavy enough armament to deal with any raider. The Inconstants were the brainchild of Vice Admiral Robert Spencer Robinson, Controller of the Royal Navy, who was responsible for drawing up the RN's trade protection doctrine. To him, there were two main threats to trade; warships, which could clear British cruisers from the sea, and privateers, which could snap up the resulting unprotected trade. The necessary response, as expounded by Robinson to the 1871 Committee on Design, was for corvettes and sloops to protect trade against privateers and smaller warships, while the frigates drove off any warship. In this statement, Robinson seems at times to be advocating for the establishment of a convoy system, but later argued that such a system would be impractical for steam merchants. This system would provide the blueprints for the RN's response to commerce warfare into the 20th Century.