I got caught out by this - completely missed there were new bank notes, had 2 20-pound notes leftover from 2016 trip. Couldn't get anyone to change for me (I don't have a bank account in England, no access to Royal post office until last day after I was alerted to the problem, and that was Oxford and the queue was....67 people long? Students just back) So I still have the old notes. Meant to give them to my friend to change at her bank but forgot. Oh well.