Another vote for 3 here.
£12.90 p/m sim only deal for unlimited data, no fair use restrictions (except no tethering), and 4G included.
Also includes 200 minutes voice and 5000 texts, so might not be ideal if you use it to actually make a lot of telephone calls so much (although you can get around that if you can use FaceTime or similar on the data allowance).
Also lets you use that in a load of countries for no extra, no roaming, and you're only tied in for 30 days.
They also claim they won't increase the price.
That's the one I've gone with, customer service has been good so far and it was easy to keep my number.