A bit off-topic: last time I tried to use a 50 GBP note in London I got strange looks and it was refused. Apparently anything beyond a 20 GBP note is seen as "too high". A worrying turn of events…
I think this is true in most countries now, in that ATMs are more and more designed to just spit out a single denomination, which raises the statistical likelihood that any larger denomination note could be counterfeit.
Some shops will do extra verification, such as rubbing it against paper to ensure the ink comes off, and use a UV light to see the hidden symbols. Even though it might be legal tender, sadly there's no obligation to accept it. Just as they might refuse Pokemon cards as payment too. https://www.bankofengland.co.uk/knowledgebank/what-is-legal-...
Wait for a few months after Oct 31st, then it should be fine.