Generally Three let you roam abroad for maybe three months at a time, with a data cap of around 20 Gbyte/month, depending on your deal.
When your monthly billing date rolls over the data may be released again, that may have been happened to you.
After that they may cut it off altogether, you are required to use you're SIM predominantly in the UK, they do keep tabs on this at least on the 'phone ones.
With a 'phone you should receive text messages warning you when it's about to happen and ways to extend things.
A brother who spends very little time here nevermind just in the EU, runs two, posts one back from time to time when it seems to be disabled, to put into an old 'phone left running to keep them happy. "Feel at Home" is invaluable to him where he goes.
If using a
MiFi you may not see such things. They are pretty good with my hotspotted 'phone and shown a lot of leniency, being a customer almost since the start. But of course I can also talk to them to sort it out, they have never cut off voice. maybe not so easy with a data only setup.
And yes, their roaming partners do vary widely in performance, manually selecting the better ones can make a big difference. As well as having a modern device that covers all the bands and technologies used in the location, an older thing may not have full coverage of everything available.
