Anybody who happens to have any of the previous design of £50 banknotes featuring John Houblon (probably few, as unlike in 50 euro notes in the euro zone they don't get handed out of ATMs) should know that they will cease to be legal tender tomorrow.
After that, they can be exchanged at branches of Barclays, NatWest, RBS, Ulster Bank, and post offices until 30th October, and after that only at the Bank of England.