There's also the British penchant for deliberately mispronouncing French words. I have heard "renaissance" pronounced "reh-NAY-sance", "fillet" pronounced "fill-it", "valet" as "val-it" and so on. I think it's a national point of pride to pronounce the words of their neighbor incorrectly.

America is at least as guilty of mispronouncing non-english words it's just natural drift.

As to fillet and valet, they joined english before the contemporary french pronunciation, and are much closer to the middle-french.

Those are the standard British pronunciations, if you meant 'I have heard' as though it might be a niche or occasional occurrence. ('fill-ay' et al. are AmE pronunciations.)

It's not always that way though, consider 'niche': it's AmE that decided it's 'nitch'!

"Valet" and "cadet" is an interesting pair: they rhyme in French (/va.lɛ/ and /ka.dɛ/), but rhyming them in English would be ... unusual.

If there were just French words pronounced in a French way and English words which came from French and are now pronounced in an English way that would be bad enough but in fact we have a whole spectrum of bastardisation.

Yep. And try "lieutenant" or "herb" on for size. (Edit: I guess "herb" is a bit of a complex one... originally from Latin's "herba" where the H was pronounced, but from UK it came most immediately from French's "herbe" with no H sound. So UK did somehow shortcut back to a more original sound.)

It's a national past time for us Brits to annoy the French. Kind of how two cousins who don't like each other would behave at a family gathering

So this isn't the British being deliberate obtuse, foreigners pronounce English words wrong all the time and we don't accuse them of doing it on purpose. They do it because that's how they would pronounce those words in their language.

Fillet/valet are mis-pronounced because of mallet, pallet, etc. Renaissance? Nail, snail, tail, etc.

It really is that simple, we're just pronouncing them as if they were an English word.