Are you sure that your father gave up his British Citizenship?. Both the UK and Canada allow dual citizenship, unlike some other countries, e.g. US, Australia, India to name a few. The fact that your father became a Canadian does not automatically mean he loses his British citizenship. I know, cos I am a dual of both Canada and the UK ! The only other possibilities are that they didn't allow dual citizenship back when applied, or, for some other reason he felt compelled to renounce his British citizenship !!
CG.
