AT&T vs Verizon.
iPad 3,2 if you have an iPad 4G model. 3,1 if it's just WiFi.
I saw 3,2 when I added it to my Good for Enterprise server and I have Verizon iPad
Actually, it might not be. The only difference between the two 4G models is the LTE frequencies that they support, both of which are useless in the U.K.
I would try the AT&T(3,2) one first. If that doesn't work, then use the Verizon(3,3) one.