What would you add?

Wouldn't mind being able to call someone who's being a jackass a 'plonker', but I think in an American accent it would sound like some kind of obscure slur

When I lived in UK my coworkers referred to idiots as muppets. Faaaahhking muppet!

This was funny every time to me.

Muppet is a good one. It's great for using in front of kids too (as long as you don't prepend it with anything too harsh. "Bloody Muppet" is safe to say in front of tweens and upwards, in my view

I didn't know these words were distinctly British.

What about knob? "He's such a knob", "Oi, knobhead!"

Most of them don't surprise me, but posh? What do other English-speakers say to mean posh?
- chic classy deluxe elegant exclusive fashionable grand high-class luxury opulent ritzy swank swanky trendy

I don't mind wheeliebin. We have a tendency to naff-ify everyday items so they always sound unglamourous - witness Brit garage "garidge" vs US "gararj"). In Australia they were called "Otto", after the company that made them.

Not sure which words I'm uncomfortable with.

I try not to use Twat even though it can still happen when I'm really exasperated.

Also loo and bog for toilet I avoid. Only bog is really off limits though.

I've used all of those except wheeliebin, which I've never run across before.

Ain't no shame in borrowing good slang, and that's something brits do extremely well imo. It's usually easy to use, rolls off the tongue, and sounds just silly enough.

I'm right chuffed about it

Keep it simple. You're chuffed about it. Not "right chuffed" otherwise you're over-egging it.
Don't forget the antonym "narked".
- That's a new one to me! Thanks, I really do love discovering new words :)
- You can say reet chuffed or dead chuffed.
Amusingly, wheeliebin isn't even slang, it's just what we call them. Like if we've had extreme winds, you might see news anchors talking in their pish, RP British accent about how people have had their wheelie bins flying away
Edit: chuffed is a good one. It feels good to say. It's more than just saying "I'm pleased with myself", because there's an earnestness to it.
- Newsreaders haven’t had to talk in RP since at least the 70s or something. They’re all pretending to be regular middle class now, even if they still went to Oxbridge.