Superdrug Mobile sim: it claims it’s a “contract”, it’s nothing of the sort – just an unlimted PAYG sim that you pay £20 per month for via a cpa so you can cancel at any time. Totally unlimited european and I think possibly worldwide roaming included. We’ve one in a dongle in the van as a mobile hotspot after our kids lunched through 60gb of data on last summer’s european trip. When we went to spain in september with the motorbikes I just tarted it out to my mates and we all used that. Its 3 at the back end, but without thier fair usage restricitons (12gb?).
You might have to mangle to APN slightly to use it abroad. Superdrug have good instructions online that I was able to follow while queuing for french customs.