There's an extremely long-standing convention of referring to timezones by the major or capital city within that timezone, so Siri should of course assume that they're talking about London in England. In fact, this is probably more or less the correct, canonical way to ask this - asking about GMT would not give you the correct answer, since England is currently on BST which is GMT+1. Needless to say, this argument would not apply to asking about anything else about London. Context is complicated.

For similar reasons, anyone who asks for the time in Boston probably means Eastern Time regardless of how far they are from Lincolnshire, though I think the more usual and canonical way of referring to that is New York time.

Pedantic: London is currently in British Summer Time, not GMT (which it is in the winter)

I'd expect it to ask the first time around, and then remember the answer afterwards.