Sy I have just returned from the States :
I set up a “Tello” eSIM (Tello is a T-Mobile MVNO) which Tello allow you to activate BEFORE you reach the States. (Tello doesn’t yet offer international roaming, but WiFi calling works well)
”US Mobile” also offer eSIM, but activation must be done in the States.
Also, Amazon ship both Tello and “US Mobile” SIM cards to UK addresses from their warehouse in Edison, New Jersey, which takes about two weeks.
US Mobile give you a choice of two networks currently - Their “Warp” service runs on Verizon, their GSM service runs on T-Mobile, and will soon be renamed “Halo”, as on 1st June, they will launch their “DeathStar” MVNO network on ATT.
Interestingly, US Mobile are planning to unveil native roaming throughout the States, with phones switching automatically between each of their three networks depending on which offers the best signal wherever you are, so, in theory at least, you should always have good coverage.
If you were to look at Tello, and decide to give it a go, let me know, I can give you my referral code and who would each get $10 Tello Dollars