If you haven't done so already, have a 2 minute read up on how federation and the fediverse works. It sounds a bit complex to start with but it's basically just a bunch of servers that all talk to each other (like how if you're on gmail you can send an email to someone on outlook even though it's a different server).

As for feddit.uk, I think it's generally got a pretty good Casual UK sort of feel. You'll find that some instances take things very seriously or have power-tripping mods, this one seems to generally be pretty chill!

The main different compared to reddit is that it's much smaller here. Don't expect an active community for every niche interest, and don't expect thousands of new posts every day. Due to how federation works you might need to use external tools to find stuff, for instance lemmyverse search to find few communities.

Ultimately it is what you make of it. Find the stuff you like, ignore the stuff you don't, and post the stuff you want to see more of!