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