Good Morning @GaryMack, if the handset is unlocked, you should be able to use any SIM card you choose in the handset.
Have you possibly researched any SIM cards that offer exclusive packages for USA roaming, that are based in the UK?
You do also of course have the opportunity to utilise Wi-Fi in multiple locations in the USA - if you're going to NYC for instance, you literally can't move without some free Wi-Fi!
With a Wi-Fi connection, you will be able to utilise apps such as WhatsApp to make calls and send messages/pictures too
Kindest regards,
David_Bn