Don't know if you've found the link but these are the tariffs for 12 months or 30 days only: https://www.o2.co.uk/shop/sim-cards/sim-only-deals/#deviceType=tablet&contractLength=P30D or bizarrely, its actually under the tablet sims section for costs: https://www.o2.co.uk/tablet-sims and the upper limit is 10GB for a topup at one time.
I did, as an experiment, try the 5GB 30 day PAYG one purely for work purposes for a couple of months. Which was just a perfect amount of data surprisingly for a 5 day week over a month just for work. But the top-up amount is expensive to continually do on PAYG though bear in mind if you're occasionally using it over a period then it works out fine as you don't lose the remainder until the expiry date if you take the right tariff for you.
But I'd suggest taking a look at the "red" network as they are cheaper for the larger data tariffs on a 30 days contract. Unlike o2 they don't do PAYG so its an advantage for o2 in that respect. I have done that to effectively replace my landline broadband and bought an unlocked Huawei Wifi Pro from ebay but that's another subject in itself!
