Virgin did a PAYGO Sim where if you went the Data Max plan it gave you 99.9 Gb (Yes 99.9) per 30 days for a £10 top up, but been a while since Ive used that one, so things may have changed now.
It all depends what your trying to do with it, in terms of Data use, as there are some tricks out there to be had.
First off, two things to consider (forgetting about 4G for now), 1, only a proper PAYG Broadband Sim is going to give access to a decent through put on speed, ie on 3G HSDPA around 5 to 8 Mb download speeds and access to Laptop / Tethering use. 2, if using a PAYGO Voice Sim for Data use your speed will be limited to around 1.5 to 3 Mb speed and Tethering is not allowed and could be blocked after a while. The only exception is Vodafone who dont care if you Tether (its your data so do want you want with it attitude) hence why Vodafone dont offer any Unlimited Plans, the other it T Mobile, who dont allow Tethering but dont seem to enforce the policy. All the others can, will or do.
Probably one of the cheapest ways is a standard T mobile PAYGO Voice Sim, where you can get Unlimited Data for £20 for 6 months (these are the one you see on the Bay for up to £40), but its not unlimited, you get a 500mb per month fair use, so after that you can still surf but no Video downloads, these Sims you can get for free from T Mobile then just top up £20 and buy 6 months Data use (PM me for how to).
There are other ways but most start to get expensive when you hit or go over your data allowance, failing that its down to a 12 month Contract Sim of some description.