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The tax treaty between the government of Canada and the government of Great Britain state that a person needs to pay tax in the resident country to avoid double taxation. So if you are a Canadian resident then you need to pay tax on a foreign pension in Canada. You need to report your gross foreign pension income received in Canadian dollars in the year. Use the exchange rate of the Bank of Canada on the day you received your pension.
Read article 17 from the link for Pensions and Annuities: Convention Between the Government of Canada and the Government of the United Kingdom of Great Britai...
Find out more about: What Is Exempt Foreign Income?