Started watching lots of videos to educate myself on tanks of WW2, there were some differences in the tanks, they were changed slightly to fit British tank practises, for example they removed the anti air 50 Cal as they felt it was superfluous but think they were all minor modifications. Maybe those damn yanks sold us tanks made of monkey metal.

Not monkey metal - we made 'em from all that tea left over from our Boston Harbour party 160+ years earlier. ;D

The American Sherman appears to be a late M4A3 model. These typically had "Wet" ammunition stowage and a thicker, solid glacis plate. The British Shermans are from BF's M4A4 kit, which had dry stowage and marginally thinner frontal armor. So it makes sense for the American Sherman to be more damage resistant than the British version, though less so for the Firefly to magically have more resilience than the baseline tank.