From Bank of England website
The Bank of England is withdrawing the older style £20 notes that show a portrait of Sir Edward Elgar. Holders of these notes may continue to use them until the end of June 2010.
For several months after the end of June most banks, building societies and Post Offices will accept the Elgar £20 notes for deposit to customer accounts and for other customer transactions. However, agreeing to exchange notes is at the discretion of the individual institution