What do you want!?
Data?
Texts back to the US?
Calls back to the US?
A few hints about the UK Cellular market:
1) You won't pay to receive calls or text messages on a UK plan - even when you're being called from another country - like the US. It might be cheaper for your friends/family in the US to look for good international rates if you are planning to contact home a lot.
2) Plans are much cheaper here. You're looking at £15 per month ($23*) for a prepaid plan that includes unlimited data as well as a lot of calls and texts (to UK numbers).
3) Plans are much simpler than in the US. You won't be forced to get a data plan with your iPhone. You just get whatever plan you choose to pay for.
Your Sprint iPhone will work with any of the UK's carriers (listed in the order of their customer base, highest to lowest):
Orange
T-Mobile
O2
Vodafone
Three
You can have a look through these pages and see what seems like good value to you.
Additionally, there is an MVNO called
giffgaff which runs on the O2 network. It offers very low-cost plans including unlimited data, but there isn't as much support available and it's a bit harder to buy one of their SIMs.
My personal opinion: O2 has excellent 2G coverage, but they did not invest anywhere near as much in 3G as other networks. The plan on offer from Three gives you genuinely unlimited data on a network that only offers 3G (they have a very limited amount of 2G roaming in rural areas). If data is important, then that seems like the best choice.
*That price includes the standard UK sales tax (VAT) rate of 20%, so you can really see how much cheaper that is compared to the US.