If you want to just pop a sim card in, you can only go with AT&T or T-Mobile. The other carriers don't use sim cards.
T-Mobile PAYG charges $1.60 a minute for calls to UK.
http://www.t-mobile.com/shop/plans/prepaidrates.aspxAT&T PAYG charges 39c a minute for calls to UK
http://www.wireless.att.com/cell-phone-service/cell-phone-plans/pyg-cell-phone-plans.jsp?wtSlotClick=1-001SJX-0-1&WT.svl=calltoaction The wording on this page is weird tho - it says 'check this international rates table for your travel destination' but I'm 80% sure that actually means 'check this table for countries you want to call from the US'. I could be wrong.
Virgin mobile (you'll have to buy one of their phones, from as cheap as $9 charge 20c a minute to call the UK
http://www.virginmobileusa.com/rates/internationalCalling.doVerizon Wireless prepaid service is $1.49 to the UK
http://b2b.vzw.com/international/prepay/index.html but you would also have to buy one of their phones.
Boost Mobile (Sprint's PAYG service) is 20c a minute to the UK
http://plans.boostmobile.com/paygo.aspx and you would have to buy one of their phones, but the cheapest one is more than the cheapest Virgin phone.
AT&T (Sim card) - 39c a minute to UK for calls; 25c per international text sent; 20c per international text received.
T-Mobile (Sim card) - $1.60 a minute to UK for calls; 35c per international text sent; 5c per international text received.
Virgin Mobile (buy a phone) (uses Sprint Nextel network) - 20c a minute to UK for calls; 20c per international text sent; 10c per international text received.
Verizon Wireless (buy a phone) - $1.49 a minute to UK for calls; 25c per international text sent; 10c per international text received.
Boost Mobile (buy a phone) (uses Sprint Nextel network) - 20c a minute to UK for calls; 10c to send and receive international texts.
Coverage is pretty good in all major cities for all the networks now, you really only see a difference when you drive between major cities. Check each carriers site for coverage maps. For overall countrywide coverage, from best to worst (although again, it depends where you are and check their maps) Verizon...AT&T...Sprint.....T-Mobile. I work in the cell tower business, I travel frequently between a lot of our rural sites out here, and have both a Verizon and AT&T phone, which pretty much guarantees I have a signal on one of them wherever I am.
Hope this summary helps a bit [:)]