I'm just back from the us, 2 weeks in Florida.
Yes, you can buy them over there, the £399 model over here is about 80 to 100 cheaper over there.
If you do buy, get rid of the box there, or maybe buy the uk adaptor before you go and takeit with you, then it looks like you have bought it here.
The IPad is 4g compliant but the uk will be using different frequencies than the us are using..... But 3G will be fine. (I'm a telecoms engineer, I know these things)
Not sure about versions, but ask for the international or GSM model that accepts a SIM card, the Verizon ones work off CDMA and have no SIM.
Walmart sell them, as do best buy, which are the easiest stores to locate while your over there.
Steve