Keith Jarrett will play a solo concert at the Royal Festival Hall in London on November 20, 2015. Tickets should go on sale today (Monday, September 28).
Update (September 28, 2015). Tickets will actually go on sale on Friday, October 2 at 10 AM. They will be available from southbankcentre.co.uk and gigantic.com. Here is the press release from Serious.
Update (September 29, 2015). Tickets will already be available to members on Thursday, October 1 at 10 AM (thanks to Stephen for the information).