I wonder why the British never came out with a
fighter based on the Bristol Hercules, particularly
from the 1,650 hp Hercules VI version onwards, when
the battle ruggedness of radial over inline engines
was already quite evident.
Too small, too late. It was never powerful enough for the first line fighters of the time, and there were better radial engines available by the time the drag problem was solved. It was also a bit on the heavy side - the Merlin was ~100kg lighter.
Furthermore, fighters of the time were designed around the engine. During the 1930s nobody understood how to streamline radial engines effectively, so fighters were largely designed around inline engines (the USN was one of the very few exceptions to this trend). Only once this problem was solved with the development of a close cowl that could actually extract thrust from the engine cooling system did radial engines become the engine of choice for fighters.
The British did introduce a radial engine fighter right at the end of the war (Hawker Tempest II), and the last British piston engine fighter (Hawker Sea Fury) was also radial engined. Both used the Bristol Centaurus engine.
There was a very neat Bristol fighter designed around the Hercules: the Type 153. It was a loser in the 1937 cannon-fighter competition which was won by the Whirlwind (it had four Hispanos under the wings). The estimated top speed was 357 mph (obviously using an early, low-powered version of the engine).
The plane makes an appearance in [b]The Foresight War[/b] as the “Beaufighter”, a naval fighter, and the “Brigand”, a land-based fighter-bomber version. 