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.

https://www.ofcom.org.uk/online-safety/protecting-children/age-checks-for-online-safety--what-you-need-to-know-as-a-user

If this law says in place and is actually enforced all fediverse instances will need to start verifying age for UK users too.

It gets you to turn your head. It’s not just a selfi.