The largest ship ever built for the Royal Navy was Vanguard. She was basicallly a lengthened version of the King George V class ships. She was ordered in 1941 with the Emergency War Plan of 1940. However, she didn't enter service till 1946. To put it bluntly, she was a complete waste of time. The age of the big-gun Battleship had passed, and she was just used as a 'status symbol', in very much the same way Hood was in the inter-war years. Vanguard used the twin 15-inch gun turrets which had originally been removed from Couragrous and Glorious when they were converted to aircraft carriers. Foo is right though, the longest ship ever built for the Royal Navy was Hood, at 860ft long......