We were twice in the UK in 2019, the first time for three weeks in February in London, Cambridge and Glasgow, then another week in December in Cambridge. TL; DR: We lived easily with credit cards and we never got any British money.
"Contactless" credit cards are widely accepted in stores and markets and on public transport. Transport for London transport (tube, bus, DLR, London Overground) accepts contactless cards and you can (if you wish) avoid getting a TfL Oyster card. Outside of flea markets, you should have no trouble accepting a credit card.
If you want change, your best bet is to use a debit card at an ATM in the UK. but not at an airport, after ensuring that the card has been issued by a bank that does not charge ATM fees abroad. Do not use machines that charge fees and apply higher than normal exchange rates. Travelex ATMs are particularly unwanted in this regard, as high fees and unreasonable exchange rates are the order of the day. Every ATM I have seen at LHR, LGW and STN was operated by Travelex. If an ATM or bank charges per transaction, a larger withdrawal is cheaper per dollar, but you have to weigh the increased risk of carrying a bigger pile of money.
Getting British currency in the US before departure is not much better; wait until you arrive in the UK and leave the airport, then use a local ATM.