It's not really comparable, almost every native English speaker can understand most British English fine, it's only when people use excessive slang or regional accents that people have issues (and that's an issue with any language - it can easily be an issue among native speakers within the UK!).

It doesn't really matter if you natively speak British English instead of American English, whereas French and English are obviously completely different languages and the switch made French a lot less useful and English a lot more useful