Jack Letts is a birthright citizen of both the UK and Canada because he was born in England (thus gaining right of citizenship there) of Canadian parents (thus acquiring Canadian citizenship). His British citizenship right would have lapsed if his family had left the UK when he was a child, but they stayed and live there still.

Canada's gripe against the UK is based on the fact that young Letts has never, in fact, lived in Canada. Our government claims that Britain's share of state responsibility is therefore greater and less shruggable-off.

Britain's centuries of existence as a state or states in various forms gives the UK precedent for yanking citizenship, but I don't think anyone has been formally banished since maybe the reign of Elizabeth I. Even prisoners transported to Australia remained British subjects.