We had an interesting exchange directly with Disney early on in the planning of this trip. When we called looking for airfare, after seeing how bad it would be booking one-way trips, they said that they could not offer us airfare. I cannot really remember how they said it, but gave us the impression they do not do airfare to Canada at all. However, something (can't remember what), made us go back and re-check with Disney. What it seems like they really meant to say was that they only offer Canada from a couple of hubs - Toronto being one of them. If we'd made our own way to Toronto, we could have used Disney. However, since we decided to extend the trip, we decided not to bother.
Could you be in the same boat (lol, same boat. I crack myself up...) I wonder? Maybe out of London it would have worked?
EDIT. Oops, I should have read further down the posts. You already commented on London...