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.