If the £20 notes date from 1996 they will feature Michael Faraday on the reverse. These banknotes were withdrawn from circulation in 2001. Fortunately Bank of England issued notes retain their face value for all time, however it is highly unlikely that any institution other than the BoE will exchange banknotes of such an age.
The Edward Elgar £20 is being withdrawn at the end of June and for several months after banks, building societies and post offices will allow these banknotes to be used for customer transactions, however once this "grace period" has elapsed it will again only be the BoE that will exchange the withdrawn notes.
Details on banknote exchange can be found here...
http://www.bankofengl...s/about/exchanges.htm