What is the "best" way to refer to those who live in the UK?

"British people", or "Britons" (less common, though shorter), or, less formally, "Brits" (that would be the slang form that you are seeking). That covers the inhabitants in general. If justified, say "UK citizens", but note that it is a legal definition, not all who live here are UK citizens - we have many here from the EU, working legally as they are entitled to, and some from former colonies etc who have not yet become UK citizens.

I have never seen or heard a reference to "UKers" - to me, as a British citizen born and bred and living here for over 60 years, that expression does not exist.

Just a reminder that the UK consists of England, Scotland, Wales, and Northern Ireland. In a few years time Scotland will probably again be pushing at the door of independence!

If you don't know what part of the UK the person is from, or if you're speaking generally: the British, Britons, a Brits; or a British person, Briton, Brit.

If you want to be more specific you should call them be the country their from: an English person, Englishman; a Scottish person, Scot, Scotsman; an Irish person, Irishman; a Welsh person, Welshman.

British is the usual term, some really patriotic Scottish may prefer Scots, Welsh - Welshman. Northern Ireland is a little more complicated. NEVER refer to them as Irish, it's a huge insult and would probably get you beaten up. British is probably the safest route with them. And Ireland, call them Irish, never British. Because they despise the UK and there's a good chance you'd be killed for it.

As British or the Brits for short

They call us "Brits" informally and British formally. Mo Brit

I think Americans refer to them as Brits.

The general area of the UK is referenced to as European

Britons or British citizens as is on the passport.

"UK citizens," "UK residents."

Douchebags.