but the Brits preferred to call it a microlight, perhaps to avoid confusion with the US'an ultralight
Nothing so sensible.
When ultralight aircraft first appeared in Britain, they were referred to here as "minimum aircraft" and the BMAA, or British Minimum Aircraft Association was formed.
When CAA started to regulate them, they needed a name, but BMAA had invested in a logo, headed paper, business cards etc. and couldn't afford to do that again. So Britain got "Microlight" aircraft instead as it also began with M.
The original UK microlight aircraft definition was MTOM up to 390kg, and a wing loading not exceeding 25 kg/mē - so different to what any other country was doing. It eventually converged with the European 450kg definition about 1999, and then the newer European 600kg definition around 2022. It's clear that Ultralight/Microlight/LSA are in the process of converging, but that's not happened yet.
G