This was a very helpful reply. Thank you.

Yes, you can use it basically like an iPad (though you'll get 'No Sim' alerts pop up). If you want to flip in a USA SIM then the answer is "very likely" you can use it.

To put a USA sim in, the first issue isn't the phone but whether your UK carrier has 'locked' the phone to your UK mobile phone plan SIM card. Some carriers 'lock' phones, some don't. If you've had luck taking your UK phone to Europe or other places and popping in SIM cards at random, then the phone is unlocked.

The second 'maybe' problem is the differing frequency for 5G services that is used. Some carriers in the USA may only operate on a specific band that isn't supported in a UK phone (though most carriers all operate on some common frequencies between both countries). For a phone / frequency list of iPhones you can look here:
https://www.apple.com/uk/iphone/cellular/

This really isn't as much of an issue as it was in the past as most carrier are using the same frequency, but it can be an issue in areas with lesser service where you don't have as many towers or choices.

Thanks from OP

Yes! you can , but it depends on your Wifi strength.

Laugh away as I am not tech savvy in the slightest

I’ve been living in the UK for the last year for work. I kept my US iPhone and service and got a second iPhone for UK service. I’ll be moving back this summer and will cancel my UK service, but the phone is in great condition and I’ll be keeping it as a backup. Can I just remove the SIM card and use the “UK” phone sans cell service for web browsing and apps if connected to wifi? I was thinking yes, kind of like how iPads work if you don’t have a data plan, but I wanted to check. It’ll be great if I can use the extra phone for streaming Netflix while being able to use my main US phone for texting