We were in the UK twice in 2019, first time for three weeks in February in London, Cambridge, and Glasgow, then another week in December in Cambridge. TL;DR: we lived easily on credit cards, and never obtained UK currency.
"Chipped" credit card acceptance is pervasive. "Contactless" cards are also widely accepted in shops and markets and — very usefully — on public transport. Transport for London conveyances (the Underground, buses, DLR, London Overground) accept contactless cards, and you can (if you wish) avoid getting a TfL Oyster card. Other than flea markets, you should have no difficulties with credit card acceptance. It's worth noting, however, that while credit card tipping is slowly becoming easier in restaurants, currency is easier to use, particularly for tipping housekeepers when you're paying for lodging.
If you do want currency, your best bet is to use a debit card at an ATM in the UK but not at an airport, having made sure the card was issued by a bank that doesn't charge an ATM fee overseas. Do not use machines that charge fees and apply higher-than-normal exchange rates. Travelex ATMs are particularly undesirable in this regard, as high fees and unreasonable exchange rates are their order of the day. Every ATM I saw at LHR, LGW, and STN was operated by Travelex. Money-changing businesses with people behind a counter, particularly at airports, are also extremely expensive. If an ATM or a bank does apply a per-transaction fee, a larger withdrawal is cheaper per dollar, but you must weigh the increased risk of carrying a larger pile of cash.
Obtaining UK currency in the US before departure isn't much better; just wait until you arrive in the UK and have left the airport, then use a local ATM.