Apparently it can be refused if the transaction is instantaneous but not if settling a debt. It all comes down to the definition of legal tender. https://www.bankofengland.co.uk/knowledgebank/what-is-legal-tender
I used to get a 50£ note in my expenses when i left home to find work in 86: the local boozer in northampton always took it and it became bit of a running joke: something like "he's here look, flash northern cant"