If you want a SIM that you can use occasionally whilst retaining service and number, the best compromise offer I found was from T-Mobile (USA).
You get a PAYG SIM. You top it up immediately with $100. This puts you into their PAYG "gold" bracket.
Once you are "gold" your minutes and service are retained for 12 months. You stay "gold" forever, unless you let it expire.
Once you are "gold" you only need to top up by the minimum $10 to extend your service and any residual credit for a further 12 months.
You can switch tariff into their $2 per day or $3 per day offerings (unlimited calls, texts and data) whilst you are in the US. You have to switch back to regular PAYG as you leave, though, to take advantage of the 12 month expiry.
You can do all the topping up and switching on-line.
With one caveat: their network coverage isn't as good as, say, AT&T in some areas. So it depends on where you are going to be.
Otherwise, as others have said, UK Network "three" has a very good roaming offer with one small-print exception: calling an overseas number (eg a USA one) is still charged at sky-high roaming rates. It's data, and calls to UK numbers, and receipt of inbound calls that are the benefit.