FFP
HMG reserve the right to tax "Government Pensions" in the UK. If you emigrate after retirement then your Pension will still be paid net of UK tax. The definition of a "Government Pension" is fairly broad and includes people like Teachers and Nurses.
If you emigrate to a Country that has a "Dual Taxation Treaty" with the UK then it can work to your advantage. Your pension will be taxed in UK taking into account the appropriate UK tax allowances. Your country of residence will not tax you on your pension but will tax you on any other income after taking into account any appropriate tax allowances in that country.
If you emigrate to a country that does not have a treaty with UK then you will be liable to be taxed on your pension in UK AND your country of residence.
Sadly, I do not believe there is a Dual Taxation Treaty between UK and Canada.