I run an encrypted XMPP server for about a dozen people. It's completely ephemeral in the sense that the server stores no messages. If you're offline, you miss them, kind of like IRC.

Will this apply to me? Do I need to ensure that no UK users are on my server?

I never anticipated this back when I set up the server. I thought that implementing strong security and privacy measures was a responsibility that I should take seriously.

I wouldn't be willing to run the server if I had to compromise people's privacy. If you don't have privacy, you might as well be on a mega-corp service.