I'm sorry to say that I feel your answer is pedantic at best, patronising at worst. Almost every website that has language options illustrates 'English' with the British (Union) flag.
Your argument that America has more English speakers than Britain is a valid one, except that Project-GC seems to use the Spanish flag for 'Spanish', rather than the Mexican one (Mexico has almost 3 times as many Spanish speakers than Spain)
As for your brief history lesson (which I don't need), you might as well say that all dialects originated elsewhere, and use different flags for them too. If 'English' didn't originate in England, then it sure as hell didn't originate in America, so why not use the Viking flag, given that you seem to want to claim the language for your ancestors? I am, of course, joking, but I do so to make the point that your history lesson has little to do with my initial question.
If the site wants to use the 'English' flag to illustrate 'English' language, I would have no real problem with that, although it'd be a bit like using the Andalucian flag for 'Spanish'.
Alternatively, you could simply rename 'English' as 'US English' and still use the American flag, which, although not ideal, would at least be geographically correct.
tl;dr: please stop being so patronising and answer my question properly. If you can't, let someone else do so instead.