Absolutely start getting rid of any paper notes now and definitely don’t hoard cash for the next trip. The good news is that you can always exchange at the Bank of England in London. The bad news is that you have to get there during business hours, potentially taking valuable trip time. There is an interesting small museum at the Bank if you have any interest in the history of money and banking.
The circular one pound coins are not exchangeable anywhere, you can leave them in donation boxes and the charity can deposit them into their U.K. bank account.
I’m not gonna fight the old warhorse of cash vs card again. But, I hope others who haven’t been to the U.K. post Covid understand how quickly the U.K. and especially London is moving towards cashless. You might get an independent place to budge on that policy, but a chain will most likely be cashless and no one on site with authority to even handle cash.