If giffgaff.com and Public Mobile had an arrangement to stock each others' sim cards travellers could get one via local delivery.
Canadians going to UK/Europe would order a giffgaff card from PM, and
UK visitors to Canada would order a PM card from giffgaff.
Both can be activated online and both companies support unlocked phones. Of course, Koodo/Telus phones will not function in Europe.