Nope don't have to do anything to the iPad.
You just have to unlock the Mifi which is one of these beauties
http://threestore.three.co.uk/broadband/?mifi=1&id=1184You can find them all over the net starting for around £45 or you can get unlocked ones ready to go for about £60 from ebay (however it is easy enough to do yourself)
It will then take any PAYG or Contract broadband sim card. I use a 3 or a Voda one in mine
That's how i use my 64Gb iPad when away from wireless. You just keep the mifi in your pocket and away you go. It creates it's own little wifi network and you can connect up to 5 devices. I often have both my iPhone and iPad on it's network when out and about