If you want to keep it in Canadian currency, the best bet is to wait until your next trip to Canada. Assuming this money is from the last series of paper notes that was retired a few years ago, there should be no issue with spending it at most stores or establishments. (Some may resist taking it, but most won't.) If it's a prior series, you may have more issues.
If you like, you can go to any Canadian bank and they will exchange the notes to modern ones for you. This may take a few minutes as they use technological methods to verify that the bills are genuine.
If you want to deal with this locally, your best bet is going to be to convert them to sterling at a currency exchange house, or to swap them with a traveler from Canada, but in the latter case, unless the traveler knows and trusts you, there may be a resistance to cooperate as there is some risk with obsolete notes.