Pre-season hardly ever sells out. I cannot remember the last pre-season game that did. Maybe in the Vince Carter
I would still buy my tickets in advanced though. Ticketmaster will email your tickets to you in a .PDF file. Just print it while in the UK and take the tickets with you to Toronto.
Also, as a precautionary measure you should store the .PDF file or Ticketmaster email containing the .PDF attachment on your phone. Worst case scenario, you lose your printed tickets and you can open/show .PDF file on your phone.