Canada and the UK accept dual citizenship (also called “dual nationality”). A person may hold both citizenships and may carry both British and Canadian passports if they so desire.
A Canadian citizen may retain Canadian citizenship, unless (s)he voluntarily applies to renounce it and the application has been approved by the citizenship judge.
The present Act makes it possible to have two or more citizenships and allegiances at the same time for an indefinite period.
Additional details can be found in the Dual Citizenship section of the Citizenship and Immigration Canada website.