I did the test before they withdrew the question about what the mains voltage needed to be in order to be British (it was phrased kind of awkwardly like that).
I chose 220, but they wanted 240 for the "correct" answer. As far as I know it's 230V +- a reasonably wide margin, which would include the 220V used elsewhere and the 240V which was the traditional UK voltage is probably still common. So the minimum, as per the wording of the question, should be 220V.
I presume they've withdraw the question after someone pointed out the mistake... I hope any official test is a little better researched.
I only got one wrong apart from that (the first one, as I've never actually read their book), so I suppose I'm allowed to keep being British.