Having recently returned from the Canadian Rockies and Vancouver trip, I can confirm that the two prebookable excursions - to Lake Louise and the evening Nature Walk - can be paid for in sterling. The other excursions, which can only be booked when in Canada and do rather depend on the weather, need to be paid for in Canadian dollars. If you don't book either of the two other excursions before you leave the UK, they will have to be paid for in C$ as well - you hand all the money over to your TM and he pays for everyone. If you go on the helicopter ride, which I definitely recommend, you can pay for that using a credit card or travel money card - you pay the helicopter company directly.
Enjoy your holiday and take lots of photos - I took over 300 on my new digital camera!
AnnF