From time to time, old banknotes are withdrawn after they have been replaced by a new version. All withdrawn Bank of England notes remain payable at face value for all time. They can be exchanged for new banknotes at the Bank of England, either in person in London or by post.
There is a different process in place for damaged or mutilated banknotes.
Click on the link for further information: http://www.bankofengland.co.uk/banknotes/Pages/withdrawn/default.aspx