Upvote:1
The recently released iPad Air and second-gen iPad Mini have the widest possible 3G/4G coverage. Either of the two (not Wi-Fi only) variants of these models will get 4G connectivity with 3, EE, O2 and Vodafone, where available.
You can check 4G connectivity by comparing the LTE frequency bands of the device to the LTE frequency bands of the mobile network operators.
Checking 3G connectivty is a bit more complicated, but each of these devices has all the necessary frequency bands, and most high-end devices sold in the US have been capable of 3G worldwide for many years.
Older U.S. iPads may only get HSDPA (not terribly bad) or even EDGE (slow) speeds on some UK networks.
TL;DR: My advice is to buy the iPad Air or 2nd gen iPad Mini from the carrier you normally use in the U.S. It will Just Work and will get the highest possible speeds when you get to the UK and insert a UK SIM card.