Even though LemmyNSFW is outside of the UK the admins of any instance will be responsible for verification of UK users because of this law. This is why .zip (Lemmy and PieFed) have geoblocked the UK. It’s a lot of work and responsibility to take on.
Here’s the post from DemiGodrick explaining this situation better than I can.
HereIAm@lemmy.world 8 months ago
I believe if the UK finds a site that doesn’t comply with the law, they will block it from their side of things. I haven’t read anything about websites needing to be proactive to block the UK if they don’t plan to comply.