My experience, as a Canadian who makes frequent trips to London, is the opposite. Everyone seems to assume that I'm an American. I've always believed that this was because the simple numbers (300,000,000 Americans vs 30,000,000 Canadians) mean that someone with a North American English accent is more likely to be American than Canadian.

I suppose that it could be that my accent sounds more American than Canadian although I have evidence to the contrary - most Americans seem to spot my Canadian accent almost immediately.

One of my favourite quotes, which supports spiregrain's thesis above, is:

Canadians are generally indistinguishable from the Americans, and the surest way of telling the two apart is to make the observation to a Canadian.
- Richard Starnes, United States journalist
quoted by Gerald Clark in Canada: The Uneasy Neighbour, 1965