If you're leaving the UK you must tell HM Revenue & Customs (HMRC). Your Tax Office will give you either the form P85 or P85(S). If you still need to complete a tax return after you leave they will let you know. 

 

If you are a UK citizen and you have left or are about to leave the UK, and want to claim back any Income Tax, use form P85.

If you are a non-UK citizen and you are leaving the UK after completing a work assignment, use form P85(S).

Visit the HMRC website for more information on the P85(S).