From the UK:
There are direct cheap flights from most major UK airports to Canada and the flight duration from London is between 7 and 11 hours, depending on destination.
From the United States:
If you are travelling to Canada from the United States and you are not a permanent resident of either country you need to be careful to satisfy the U.S. authorities on any subsequent trip that you have not exceeded their limits on stays in North America. Your time in Canada counts towards your maximum allowed United States stay if you are returning to the U.S. prior to your departure from North America.