BBC article about 20 and 50 pound notes

Don't know who has all the £50 notes. Drug dealers?
Think I have only seen one once, that was more than thirty years ago

It is important to remember that if you have a supply of any of these notes that you brought home from a previous trip, you can still always exchange them at the Bank of England in London even past the deadline listed in the article for their use. I had to do that with some 10 pound notes a few years ago and it was quick and painless.

You can also mail them in to the Bank of England. See the directions here. https://www.bankofengland.co.uk/banknotes/exchanging-old-banknotes

Also be careful if you change money before your trip, there have been tales of people getting stuck with "old" money that they cannot then use in England. Not a good use of time to have to visit the BoE to get plastic money.

An old article, but still a nice reminder to switch to the polymer currency. I exchanged my remaining £20 notes earlier this year. I exchanged the £5, £10, and a few £50 notes (my sister still had after a trip) a few years ago. I was even able to exchange a few old £1 coins just after the transition period ended. If you happen to have any old £1 coins now, best to toss them into a donation box, give to somebody with a high street bank account, or into the trash.

Don't know who has all the £50 notes. Drug dealers? Think I have only seen one once, that was more than thirty years ago

Some ATMs inside banks do dispense £50 notes these days, although they usually have a sticker on so you know, either to pick that one or to avoid.

It is important to remember that if you have a supply of any of these
notes that you brought home from a previous trip, you can still always
exchange them at the Bank of England in London even past the deadline
listed in the article for their use. I had to do that with some 10
pound notes a few years ago and it was quick and painless.

I went there to exchange some old £5 notes a few years ago. I'd never explored the area around the Bank so I took advantage of the task to have a little wander around. Just around the corner from the entrance to the Bank is the (free admission) Bank of England Museum. I'm glad I visited. Among other things, you can pick up a gold bar* - and yes it's heavy. All told it was a pleasant morning and a good memory from that trip.

*Edited to add: you can lift the gold bar, but you can't carry it away. Just saying. :-)