Nobody has mentioned availability yet. When designing a new aircraft type, you go to your (preferred) engine supplier(s) and see what they've got on the shelf/drawing board and how this fits your needs. Wright and P&W had a lot of radial types available or could produce them in sufficient numbers. RR had the Merlin available and could produce these in sufficient numbers. Other types may not have fitted the design as well as these did.
(I know that I am over-simplifying this... but it helps to get the point across. Nobody ever sat down and decided to only have radial engined fighters in the US and inline engined fighters in the UK, so the original question is also very much simplified, as already shown above.)
Last edited by Jhieminga; 24th Nov 2022 at 08:25.
Reason: silly typos