This answer is largely taken from this previous discussion on the topic.

There were two ships in this class, the Marshal Ney and the Marshal Soult. They were monitors, constructed at Palmer's Shipyard in Jarrow between January and November 1915. Monitors were small ships intended for shore bombardment, and were thus heavily armed for their size. Soult and Ney were armed with a single twin 15in turret, freed up by the decision to turn Repulse and Renown from battleships with four such turrets to battlecruisers with three. Other British monitors were armed with 12in or 14in guns, along with smaller ones armed with 6in or 9.2in guns. Most of the RN's monitors of the time were given numbers for names, but many of the larger ones were named for generals. This reflected their role in supporting troops ashore.

The first monitors to be used by the RN during WWI were three lightly armed river monitors, being constructed for Brazil in British yards in 1914. These three were requisitioned and (somewhat naturally given their role), given the names of British rivers - Humber, Mersey and Severn. The first to be constructed by the RN during the war were the Abercrombie class. The genesis for this class came following a visit to the UK by Charles M Schwab, president of the Bethlehem Steel Corporation. His company had been building four twin 14in turrets for a battlecruiser being built in Germany for Greece. With the British blockade of Germany in place, there was no chance that this ship would be completed, and Schwab sold the turrets to Britain to recoup his investment. Following a very quick, simple, design study, four monitors were put into construction. Initially, these ships were to be named after figures from the American Civil War, commemorating the origin of the turrets (and of the namesake of the type) - Ulysses S Grant, David Farragut, Stonewall Jackson and Robert E Lee. However, the Americans pointed out that this might be politically tactless given American neutrality. As such, they were renamed after British generals. The next class, the Lord Clives, used turrets taken from decommissioned British battleships, and were named from the start after British generals. The two Marshals came next. They were named after French generals of the Napoleonic period as a way of paying tribute to France's contributions to the war. While British ships could not be named after American figures for fear of showing an inappropriate closeness between a neutral power and a belligerent, the same argument did not apply between two allies. Later large monitors would again be named after British generals, while smaller ones were just given numbers. Oddly, though, one of the Lord Clive-class would be named after the German/Austrian general Prince Eugene of Savoy. While Eugene had commanded alongside the Duke of Marlborough in the War of the Spanish Succession, the Austro-Hungarian Navy also had a ship named after him.

Sources:

Big Gun Monitors: Design, Construction and Operations 1914-1945, Ian Buxton, Seaforth, 2012

Warrior to Dreadnought: Warship Design and Development 1860-1905, David K Brown, Seaforth, 2012