This is from the Bank of England's home page:

"Some banks, building societies, and Post Offices may still accept the most recent of the notes withdrawn from circulation for deposit to customer accounts or exchange for current series notes. However, agreeing to exchange these notes is at the discretion of the individual institution.

All Bank of England notes from which legal tender status has been withdrawn remain payable at face value forever at the Bank of England in London. Any such notes may be presented for payment either in person during business hours, or sent by post (at the sender’s risk) to Exchanges, Bank of England, Threadneedle Street, London EC2R 8AH."