hi i had an ipad 2 3g and used a 3 month sim card (3 gig or 90 days - whichever comes first) on the 3 network. i have to say reception was amazing. the cost for this is about £12.
alternatively buy this:
3 Original Broadband Ready to Go 1GB Preloaded Data Sim Card for Mobile Devices: Amazon.co.uk: Computers & Accessories
but you will need a sim cutter. or in argos 3 sim cards for ipad are half price atm which is £5 a month.