While London and perhaps much of the U.K. is going cashless, I still like having some cash for backup.
The Bank of England has recently issued a new polymer £20 note. We are currently in a transition period during which both the old and new notes may be used. Eventually the old notes will no longer be accepted. The BOE will give at least 6 moths notice of that date.
If you want to keep notes for future visits, it’s probably best to sort out the new ones. Otherwise old notes can be exchanged for new ones at the BOE in London. Be sure to checkout the gold bar in the museum if you go.
https://www.bankofengland.co.uk/banknotes/polymer-20-pound-note