I heard in the past, that an alternate word for condoms in the UK was a "french cap"-- and the french called condoms --(no attempt here to put it into french) "english letters"

I've never heard 'French cap'. I know 'Dutch' cap for a diaphragm and 'French letters' for condoms.

French letters / letters anglaises is part of a special subset of your other point about 'all Greek / all Chinese / all Hebrew to me'. What the British call 'taking French leave', the French call 'filer à l'anglais' and so on.

I used to know another couple of these English-French pairings - can anyone help out? Is belMarduk still with us?