It'd certainly be interesting to see how people got on with captchas where you identified plain-looking vehicles as taxis because they were obviously Hindustan Ambassadors. Or whether guesswork alone was sufficient to identify unfamiliar terms like 'layby' or'sleeping policeman'

Maybe. But I think the better question is whether there is any evidence that anybody is actually confused what what a taxi is to the point where they are unable to answer these things. I am skeptical.

I understand people being annoyed by US-centrism, but that is a different matter.