Q: Using UK iPhone in US

Have you considered  3 Network no roaming charges in US ,I believe

The sim you refer to would require your iPhone to be unlocked ie not UK Carrier network locked

make sure you get O2 to unlock the iPhone

If you are not aware this is how it works

How to unlock your iPhone for use with another carrier - Apple Support

No, but T-mobile will have some cheap plans, coverage not so great unless you are around a big city. Att has good coverage but has capped data and overage charges out the boot. Verizon great coverage, has holes here and there, but again caps and overages out the boot. Sprint like tmobile but improving but they wont accept any phone unless it is activated on there network.