Yeah, with the guys here, I'd seek legal advice on this one, or at the very least from Citizens advice...
My opinion though, is that its b******s. Ive never had to tell them my millage, only just to give an estimate of what I think that I will do. and, further to that, if you were bound extra charges for going over a milage limit, then they should have made that very clear to you when you took out the policy.
I've been with Highway for a long time, though, through different brokers, and I've never even been asked for current mileage.