From a legal perspective...
The requirement to have age-verification in place applies to any person who "makes pornographic material available on the internet to persons in the United Kingdom on a commercial basis other than in a way that secures that, at any given time, the material is not normally accessible by persons under the age of 18." (s14(1) DEA 2017)
It does not matter whether that person is based in the UK or elsewhere.
An ISP can be compelled by an administrative (non-judicial) blocking order to take the steps either specified in the blocking notice, or else as "appear to the provider to be appropriate", to "prevent persons in the United Kingdom from being able to access the offending material using the service it provides". (s23(1))
The legislation expressly permits overblocking: "The steps that may be specified or arrangements that may be put in place ... include steps or arrangements that will or may also have the effect of preventing persons in the United Kingdom from being able to access material other than the offending material using the service provided by the internet service provider." (s23(3))
"Pornographic material" is defined in s15. It's too long for me to paste here, but it covers quite a lot, with an emphasis on material which was "produced solely or principally for the purposes of sexual arousal". And, since different people like different things, that potentially covers quite a lot.