Could a UK exit tax apply to departures that happened before the law changed?

Retrospective taxation is unusual and generally controversial in the UK. Exit taxes elsewhere are normally triggered at the point of departure, not applied years later to people who have already left.

If you are already non-resident there is a strong argument that a new exit tax should not apply to your past departure. However, tax policy can change quickly and the detail would only be clear once draft legislation is published.