I think I'd agree with that. Well not the northern bit, I'm about as northern as a biscuit, just a bit crap at being southern (I am neither posh nor a shandy drinker, and only a part-time poof. And more seriously, my mum's family were Irish).
But if I were going to stick a flag on something (say, while touring overseas), it would be a Union Flag, not a St George's Cross.
That the English flag is usually associated with football and the extreme right doesn't help, of course, but it wouldn't really be an issue in that sort of context.