https://www.gov.uk/voting-when-abroad
If you’re moving or living abroad
You can register as an overseas voter for up to 15 years after leaving the UK, as long as:
you’re a British or eligible Irish citizen
you were registered to vote in the UK within the previous 15 years (or, in some cases, if you were too young to have registered when you left the UK)
你連uk 都無住過....就算係citizen離開超過15年一樣無得投