I have used giffgaff for several recent trips. You start by putting some credit (I remember it being 10 pounds GBP) in your account and then can choose a PAYG plan or get a 30-day plan (called a Goodybag) with the data expiring after 30 days. For the amount you seem to be seeking, you'd have a cost of 6-8 GBP for the month you travel. When your Goodybag expires, you don't need to purchase another Goodybag immediately, and a text or a call every six months (using the credit you originally deposited in your account which doesn't expire) keeps your SIM alive so that you wouldn't have to pay anything between your yearly trips and then could recharge once in time for your next trip.

Note that giffgaff has increased their data allowance for each of their Goodybag plans over the past couple of years so that the base amount could possibly rise from 500MB. I also like giffgaff (a MVNO that piggybacks on the O2 network) because they allow recharging with a non-UK credit card. And while UK-based, roaming in the EU, including Belgium and Spain, is charged at the same rate as use within the UK--unless you run over, in which case, giffgaff is not for you because the rates become extremely expensive. It's one of the reasons I buy the 20 pound GBP Goodybag which has an overall 80 GB limit but a 20 GB limit in the EU. I've never exceeded 10 GB for a trip of roughly two weeks.

More information here: https://www.giffgaff.com/sim-only-plans