Sim only for use In spain / UK

On my recent visit to Spain, Lebara worked flawlessly.
Could be a good option

...wanted a permanent number to use for regualr trips back and forth...

Cheapest non-contract off-the-peg (non-promo) solution to satisfy that requirement would likely be Smarty as suggested by @jamie15
£8/m no contract no credit check provides unlim mins+text and 12GB data, where 12GB data provides a buffer over your envisaged 10GB requirement and 12GB is coincidentally the roaming FUP limit.
Potential negs rather than choosing 3 PAYG directly:
some will argue that as Smarty is an MVNO it has less network priority than 3;
3s roaming FUP of max "continuous period of 60 days in a rolling 12-month period" may be more rigorously enforced by its MVNOs, inc Smarty.

smarty.co.uk/pla…ata

This sim only megathread is a good start The hotukdeals SIMO (Sim Only) Deals Megathread (Part 3)

£8.95 shifted directly by Amazon for 30days unlim calls/SMS + 10GB via 3 PAYG SIM+bundle that can all be used in UK / Spain:
amazon.co.uk/Thr…RX/

Not clear if OP wants a burner SIM or semi-permanent mobile number or just some access to UK number calling & some data for either
one-off visit to Spain or
ongoing flitting between UK & Spain (3 PAYG has generous UK/roaming ratio before fair use violation).
If burner, just buy as many 10GB bundle sims is envisaged and turf each one when each 10GB is exhausted.

Sim(s) must be activated in UK prior to roaming, so the months' allowances start from date of UK activation.
One bundle per calendar month.

Subsequent optional monthly bundles are about £10 for 16GB (max 12GB roaming/m regardless of any higher bundle limit) assuming the "auto-renew" promo is still running which it should be as 3 is still actively promoting it (May 2023), archived for ref here: archive.is/48TM5
with detail also archived May 2023 here archive.is/bFPsv
Check if still active & how to qualify.

I'm with plusnet and I could roam with all my allowances from home in Tenerife. Only got 12gb for around £8.

Tesco mobile payg rocket pack either 50gb for £15 or 100gb for £20,with Tesco Clubcard. You let the tariff rocket pack die after 1 month (don't record any debit card on the app to avoid automatic tariff renewal) and then you can use it again by topping up and by reactivating it another time in the future. 5G over half of Europe for me last Summer. (edited)

I was just in Lanzarote and I bought a Lobster Mobile sim out there with 80Gb of data and Unlimited calls and texts in Spain, to the UK and other countries. It cost 20 Euros
The plus thing about it is that it is aimed at the Expat community out there so English CS staff, really helpful as well when I called them.
Lobster mobile Large Plan
I was really impressed with it and you can also use it in the UK.

Choose from Lebara who let you use up to 30gb providing your plan has that amount of data.