I have never been involved in the operation of UK piston engines, but I was always told that the main difference was that UK engines were "boost limited" whereas US engines were not. What I understood that to mean was that the pilot was in charge of max boost on a US engine, and it was possible to overboost the engine by advancing the throttle. The UK engines reached max boost and were automatically limited, permitting no overboost. How all that was achieved is a mystery to me, but maybe some UK engineer will remember.
On the MAP issue, my only experience of windmilling was at cruise ( around 33" MAP on a P&W R 2800 cb3/4 (single speed supercharger) or cb15/16 (two speed supercharger), and scavenge back pressure would have been close to atmospheric so no noticeable change in MAP took place.
Possibly at high altitude in "high blower" (CB 15/16) the low outside pressure would affect scavenge back pressures and thereby indicated MAP.
I am trying to remember what happened on DC-6 when RPM switches were toggled up for approach. If my memory is correct MAP decreased with increasing rpm and vice versa. (Throttle not moved)
Thing is, it is all back in early 70s, so it is a bit hazy.