Americans who crossed into Canada and made their way across to England and the RAF lost their US citizenship, and technically became fugitives from justice.
Ah! So there weren't any actual Americans in the BoB at all then... Just a couple of Canucks and a few stateless asylum seekers...
Michael Caine portrayed Brian Kingcome
Michael Caine was playing a Squadron Leader who didn't stand a chance once his black labrador wistfully watched him taking off. Whereas Brian Kingcombe was a Flight Lieutenant who served as a Flight Commander in 92 Sqn at the time and was never shot down and killed. Thankfully he was stilll alive and kicking last time I looked.
There's a lesson there for Fighter pilots - Never but never let your pet labrador wistfully watch you taking off.