The region is tied to your Roku account (not the device) and there's no way to change it. If you no longer need that Canada account, close it and create a new Roku account. When you do, make sure you clear your browser cookies, use a different browser, or use a different device or the data saved by your browser will just get you a new account associated with Canada. In addition, make sure you're not using a VPN or anything else that may mask your true location. You'll have to factory reset the device to link it to the new account. If you travel back and forth, you may want to have both a UK account and a Canada account. If you do, keep in mind that subscriptions through Roku are tied to the Roku account. Subscriptions directly with a provider don't suffer from that problem and can generally be used on any device.
How to close your Roku® account | Official Roku Support
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