Definitely don't change money in the US before going - that will get your a poor rate of exchange.
Chances are you won't need cash at all in the UK - virtually everywhere takes card, and most places prefer it. Some places are card only. You will definitely get raised eyebrows if you try to pay cash. Even market stall vendors have card machines. Where else in Europe are you travelling? Most countries mainly use card, but some are a bit more cash based.(Austria spring to mind).
If you really do want cash, taking it from a cashpoint machine (ATM) will give you the best rate.
Revolut or Wise is popular among travelers, but only you can say whether it will offer better rates than using your credit card. My Singapore bank offers similar (sometimes better) exchange rates, no foreign use fees for credit or debit cards, and allows me to hold deposits in multiple currencies, so while I have both Revolut and Wise, I normally don't use them. And of course with a credit card you have more spending flexible than a debit card.