Just returned from London where I exchanged old currency for the new notes. When entering the Bank of England (Bank tube stop) the guard will ask the purpose of the visit, will ask to see the currency, and check any purse or backpack for explosives by placing it in a machine. You then enter the bank, go up to a teller, and fill out a paper. The entire process took about 10 minutes. The Bank of England does NOT exchange the old £1 coins. I was able to exchange the 5-£1 coins I had at a bank although not all banks will. I went to a couple different ones until I found one that would exchange them. The old £10 notes can still be used until sometime in the spring, but I exchanged all of mine. The £20s won’t change until 2020.
Open 9-4 Monday through Friday, closed on bank holidays. One can exchange up to £999 without identification, but you may want to have ID with you just in case.