Typically, iPads from UK carriers are locked... and only the original owner can get an unlock processed - and its not an easy process at all when dealing with EE. You might be able to find a service on eBay - but there's no guarantee it'll work, and for EE unlocks you're looking around the £100 mark.
Realistically, you can either sell it, and buy an unlocked one - or move to EE for the data plan.