The best eSIM for the USA
The United States has the widest gap between what a travel eSIM costs and what your own network charges to roam. UK roaming rates for the US are usually painful, and a travel eSIM is almost always the cheaper answer. The question is which one.
Coverage is about which network you land on
American coverage is a network question rather than a provider question. The three networks are AT&T, T-Mobile and Verizon, and every travel eSIM sold for the US rides on one or more of them. Coverage is good in cities on all three. It diverges in rural areas, on long drives and in the national parks, which is exactly where you are most likely to need it.
If your trip is a road trip, check which network the plan uses before you buy rather than comparing headline prices. If your trip is New York and Chicago, it barely matters.
Unlimited, again, is a daily allowance
Airalo's US plans are unlimited-only now, with 3GB a day at full speed and 1Mbps after that, and tethering is not restricted. Saily gives 5GB a day. Ubigi caps the plan rather than the day. The pattern is the same as everywhere else, and the differences are large enough to change which plan is actually the best value once you account for them.
Getting a US phone number is the hard part
Most travel eSIMs give you data and nothing else. That is usually fine, but the US is one place where it bites: some restaurant bookings, ride-hailing verifications, delivery services and hotel systems want to send you a text, and a UK number roaming can receive those while a data-only eSIM cannot.
There are three ways round it. Keep your home SIM switched on for texts and use the eSIM for data, which is what most people do. Buy a plan that includes a US number, which a small number of providers offer, sometimes as a paid add-on. Or use an internet calling app with a US number, which works for most services but is rejected by some as a virtual number.
One thing to check
Confirm your phone supports the right bands and that the plan covers 5G if you care about it. An older handset bought outside the US can end up on slower networks even where the plan itself is fine.