Buy a 3GB Three mobile PAYG sim from
Amazon at about £10.80 inc free p&p. Lasts 3 months or 3GB data. At end, throw it away and buy another.
Just need to trim it to fit iPad Microsim. I use a cutter that cost about £3 but pair scissors and bit of care will do.
Or walk into any Three store and buy a MicroSim one for £20.50. They also do a year/12GB one for about £70.
I'm a heavy iPad user and get through about 1.5GB in two week break. Never had one not time expire.