// All our currency notes contain a declaration: "I promise to pay the bearer on demand the sum of £x", signed by the govenor of the Bank of England. Even when a particular note ceases to be legal tender that promise is still valid and the note can be exchanged at a bank. //
Not necessarily "exchanged at a bank" but certainly exchanged at the Bank of England.
Other banks may refuse or use their discretion in regard of withdrawn banknotes, however all withdrawn Bank of England notes remain payable at face value for all time.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.
http://www.bankofengland.co.uk/banknotes/Pages/about/exchanges.aspx