Prepaid Data Sim for iPad

I've been using a pre-paid data sim (bought from Amazon on the Three network) which had 6GB and lasts 12 months. It's coming up to the end of it's validity, so I'm on the lookout for another deal.

On O2's website they have a 12GB deal for £40 which lasts 12 months (or whenever the data is used up) but it says that it will not work with all devices. Specifically, "Got an iPad Pro 11 inch, iPad Pro 12.9 inch, iPad Mini or iPad Air? Our preloaded iPad sims don’t work with these models."

I'm using an iPad mini 5 (new model) but can't work out why the device type would matter, unless they don't support nano-SIM sizes. I'd ideally like to use O2's network (directly or via another supplier) as they have better coverage in the areas I'm likely to use the tablet, so would appreciate any information about deals people may have seen.

I'm not interested in a monthly payment (even if 30 day contract) or a 30-day expiry on a pre-paid SIM as these work out much more expensive for my occasional use use-case.

Thanks :-)