I noticed that the Bank of England has announced that the paper notes can be used until 30 September 2022. If you keep a stash of banknotes for future trips you may want to cull out and keep the polymer notes. After that date you will need to go to the Bank of England in London to exchange paper notes for the polymer version.
On a side note I have noticed that in London you can get by with very little cash. I plan to reduce my holdings the next time I am there.