Be aware that AT&T has discontinued its 2G frequencies earlier this year
Voice, Text and Data will only be using 3G frequencies.
Check that your phone can use the 850 and 1900 Mhz Frequencies in 3G, otherwise your AT&T sim will be useless. (Your phone may accept those frequencies in 2G but not 3G. 2G is discontinued by AT&T)
If your UK operator uses AT&T for roaming while in the US, it will also not work if your phone does not accept those frequencies in 3G