Which number series is best for a Canadian to choose?

I think BT may have given up providing phone cards. There are other phone cards available. Voipuser may well be the best deal if you take everything into account and only calling the one number in Canada ... dialing is then simple. There are a number of companies that use premium numbers for their services. Search for this
International call from UK to Canada

and you can see some of them, You might be able to use one if calling from a relative. I am not clear what the actual costs are.

Added: Calling from a relative it might be simple to use a computer to use web callback such as Jajah. All you have to do is type the source number and the destination number. UK to Canada is about 3 cents a minute.