Hi Ted,
Are you flying directly from UK to Canada? If you are don't read anymore.

Or are you landing in US (just to change planes/flights) before continuing to Canada? And then entering US later.

I did the second option about 11 years ago and it made a big difference to entering US from Canada later on that trip.
Your 'visa waiver' clock starts at your first touchdown in US so you could find all the time used up (is it still 90 days?) by the time you enter US from Canada.

In those days there was a special procedure for visa-waiver passengers called "Transit without visa" or TWOV, so your ticket and passport were handled in a special way at your first touchdown in the US. Then your 90 days starts when you go back to US from Canada.

I've since heard that TWOV isn't available anymore.

I also have a funny feeling it would be worth trawling the visa waiver website to make sure any time spent in Canada doesn't come off your 90 days when you enter US. I remember there were all sorts of different rules for visa waiver travellers if they entered US from either Canada or Mexico.
It will all have changed by now I'm sure, maybe even more complicated, but I'd check.

Or maybe someone else (UK passport) who's done that route recently (air to Canada, overland into US on visa waiver) can confirm.

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TTR250 - London to Cape Town

Sure have... Great film. I don't intend to end it all in a trailer though

This is also worth watching..

The Other Side - 4oD - Channel 4

The plan is to fly direct to Canada. Thanks for the info all the same.

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Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.