After Brexit happened, UK’s mobile operators didn’t immediately introduce roaming charges but they are now gradually rolling those. The roaming charges will take place starting January 2022. O2 and Three are meanwhile going to limit their fair use caps – the former will limit the data roaming to 25GB a month while Three is reducing the limit from 20GB to 12GB.
For those of you are unaware, the EU has banned roaming charges within the EU but it has also introduced a fair use agreement where you can’t buy a mobile data plan in one EU country and use it endlessly in another.
In any case, the EU and the UK will encourage operators to have “transparent and reasonable rates” for roaming.