Most banks have the necessary equipment to cut an international MO, or convert the CAD to GBP. It is safe to send cash to these sites, I am sure a good majority of their seeds are sold on a cash basis.

Spend a little extra sending them the EXPRESS method (in the US it is trackable) I don't know for Canadian mail.

You could also call them up to see when they will be getting new stock in....might save a trip to the bank