Is this nitpicking? Some people think so.To me this is really no different than saying the Texas flag represents The United States. Yes, it is a flag of a state of The United States, but it is not the flag of The United States. Not all citizens of The United States of America relate or affiliate themselves with the flag of Texas. Neither do all citizens of the UK relate to the English flag.
It's probably an honest mistake, but funny all the same, considering the original Cash Cab was created in The United Kingdom! Respect for people's flags, and knowledge of geography beyond your own country can really prevent expensive dental bills. Don't believe me? Tell a Scot he's English and then count your teeth when he's done punching them out of your face.