I have in the past used a Raphaels Bank cash machine in London using my Revolut card. Just make sure you refuse DCC (i.e. make sure you select to have your account debited in USD) and you should get pretty good rates.
To be honest, though, at least in central london the competition is really strong for USD exchanges, and you get pretty decent rates almost anywhere. I did that mostly out of interest, and because such a machine is in walking distance from my office. I would’ve gotten almost the same rate at the nearest beareu (at least sufficiently close that even for a few hundred pounds the difference would’ve been smaller than the tube fare had I not been able to walk).