The sim can be activated outside the Uk, but usage seems to be capped at under 20gb, although I think this is people using it for streaming etc. I find I can get the full 24gb as I don’t stream.
Most of these sims that offer roam like home work on the basis of usage for short holiday or short business trips abroad. If the sim is not turned on again in the Uk after a trip away, it continues to assume that it is abroad and can (if my memory is correct) stop working after 90 days under their fair usage terms.
Davy