Verified answer

"After 28 February 2001, bank branches will continue to accept these notes and they will, of course, remain payable at the Bank of England for all time."

http://www.bankofengland.co.uk/publications/news/2...

Despite being withdrawn from circulation almost 10 years ago, the Bank of England seem to suggest that they still have a value. Of course, you cannot spend them in the shops, and your own bank might be reluctant to take them off of your hands, but try contacting the Bank of England and see what they suggest.

Michael Faraday 20 Note