They'll get blocked in the UK. Which, to be honest for a non profit fediverse instance isn't going to be a bad thing as they'll loose traffic alongside any donations. Expect UK based instances to start doing something for their users. But there's lots of other verification options they could use.
I can't see why that would be the case. The UK users using the UK instance would still have had their ages checked. As far as I understand it doesn't matter where the adult material on the site has come from. It's not a law to try to protect against the creation of material by people who aren't verified, but rather try to reduce the amount of under 18s accessing material.
I seem to remember only <7 instances allowing porn posts when I joined up. As a practical matter, I don't remember the last time I saw porn posted from any instance besides lemmynsfw.com
TL;DR: Porn on Lemmy is as centralized and easilly squashed by regulation as anywhere else.