Yes, there is a tax treaty between Canada and the UK.
If you are still considered a resident of Canada, you would enter your foreign income on the Foreign slip. You may also be eligible for the Foreign Tax Credit.
If you are a non-resident, you can enter your UK income on the Statement of World Income. You will not be taxed on this income because of the tax treaty, but it still may be used to help determine what credits or benefits you are eligible for.
