I must be missing something. I can't see this

I wonder if you need to be a Virgin customer?

Might be a bit more of a hassle, and not include the data rollover functionality; but surely this is a better deal:
http://www.ebay.co.uk/itm/EE-PAYG-4G-Data-Sim-Card-Preloaded-With-6GB-500MB-A-Month-For-12-Months-/262819238464?hash=item3d313f5240:g:khIAAOSwEjFXfhc0

I currently use one as a backup for when my phone plan runs out of data; has been working fine for the past 7 months. Can buy two of those Sims for less than the above deal.

Can get 250 mins 5000texts and 1gb from I. D mobile for £5/M 30 day rolling contract.

4g network.

Excuse my ignorance but are these any good for a cellular ipad?

I currently use mine in an EE Osprey, and that hasn't been blocked yet. I would assume that it would work in an iPad too, but don't quote me on that; your best bet is asking the seller.

Remember to keep an eye on BT. Currently on their effective 62p per month deal from six months ago for 4gb (£10pm with over £110 in Amazon vouchers and cash back). All paid out perfectly within a couple of months.

Here's a link to Virgin's data rollover FAQs in case someone has not come across this before. It's been years since mobile providers had something like this, and only for minutes and texts if I remember this well.

Three I think have one of the best no hassle data deals at present. However if the OP can't get network coverage then this point is mute. For others that may be interested you can get 12gb of data for £23. This lasts a whole year so think of it as an annual SIM rather than a monthly SIM. It can be tethered and used in a tablet. Once the 12 months is up or all data is used then just pop in another SIM from same deal as this is cheaper than topping up the old card. So just to confirm there is no monthly allowance with this SIM, it is not 12 months at 1gb per month, it is one years worth of 12gb data, use as little or as much as you want to use each month. You can receive calls and texts to the SIM number however do not try to add credit to these Sims in order to use for voice calls. It will convert from a data sim and hence become a normal monthly SIM. This is a data sim and data sim only.
https://www.mymemory.co.uk/SIM-Cards/3/3-PAYG-4G-Trio-Data-SIM-Pack-Preloaded-with-12GB-of-Data-for-Mobile-Broadband
Three also have a 24gb SIM deal but I can't see any available from stockists at the minute. This works out cheaper per GB and will likely come back in stock over next few months.
These sims can also be used abroad and are great to use for data in dual Sim phones together with a bargain voice and text SIM in order to reduce your monthly mobile bill greatly.

Yes, this was a great deal: http://www.hotukdeals.com/deals/three-payg-trio-data-sim-preloaded-with-24-gb-data-28-40-amazon-2543727
Pity it's no longer available...

[pedant mode] it's a moot point, not mute point. [/pedant mode]

The 12gb preloaded is better all-round more flexible unless you specifically need Ee network or 2g functionality which would be too slow anyway. Cold from me

Cold from me the 3 preloaded sim is £22.50 one off payment and lasts a year, no credit checks or contract.