I get the impression that rather more than a quarter are in fact supporting every team that England comes up against - some people were just a bit shy of admitting it to the interviewers. There's certainly a bit of ill-feeling about the appearance of St George's flags here and there (though very few around here, to be honest). Some of the flags are on vehicles from England, passing through for business or pleasure. It seems a bit hard if they can't fly the 'wrong' flag. In any case, you certainly see a few Welsh flags in London whenever Wales is playing England at rugby, and that seems fair enough.
But I have to admit to a surprisingly strong feeling of resentment on the rare occasions I see an English flag in Wales. This is rather curious, considering that I spent the first 60 years of my life in England and have no no Welsh ancestry, as far as I know. That's the power of the Dragon for you.