I have an 3G ipad brought from the USA, there are no differences compared to the UK model except the removable wall socket for the charger is different.
I currently have Vodafone micro sim card in my ipad and it will also work with all the UK networks that offer ipad data plan.
I don't have any issues with apps and ibooks from UK itune store, I think the ipad set to whichever country your itune account tires to when you active it the 1st time.
My ipad does show up as US model when I put the serial number in online, but the warranty is valid in the UK.
You can find a quick comparison of different ipad plans here:
http://www.t3.com/feature/ipad-uk-3g-pricing-how-the-networks-compare
There are also two things you need to consider when buying the ipad from the US.
1st the price you see on the US Apple website does not include state tax which I think is about 4% to 6%. As far as I can found out, there is no easy way to reclaim the state tax as a visitor when you leave the country.
2ed, you will need to declare the ipad when you come through UK Custom and pay import and VAT on it.
So overall, the saving will not be as much as the difference between the two prices listed on UK and US Apple store.