From the bank of England website:
The next £20 note will celebrate Britain’s achievements in the visual arts and we would like the public to nominate who they would like to feature on the back of the £20 note.
Visual artists include architects, artists, ceramicists, craftspeople, designers, fashion designers, filmmakers, photographers, printmakers and sculptors.
The Bank will not feature fictional or living characters, with the exception of the Monarch, who appears on the front of our notes.
Please make your nominations by 19 July 2015.
http://www.bankofengland.co.uk/banknotes/Pa