They will be progressively withdrawn from circulation, and when a certain %age have been returned to the bank of england a date will then be announced after which they will cease to be "legal tender"
Some banks, building societies, and Post Offices may still accept the most recent of the notes withdrawn from circulation for deposit to customer accounts or exchange for current series notes.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All Bank of England notes from which legal tender status has been withdrawn remain payable at face value forever at the Bank of England in London.