[voakbasda]

If I never plan to visit the UK, then I would expect to be able to ignore the lawsuit entirely, right? Or does this represent a new threat where the US would extradite its own citizens for trial in the UK? Would some treaty bind me to a default judgement obtained in my absence?

[nullc]

It's a hard question. UK civil judgements are generally enforceable in the US (and in most other countries). There are specific cases where they aren't (e.g. the SPEECH act protects US persons from UK libel lawsuits) and you could fight the enforcability in your local court.

A contempt ruling wouldn't generally be enforceable outside of the country, so even if the UK would send you to prison for ignoring their civil judgement avoiding the UK can be sufficient, but because damages may be enforceable you could potentially exposed to your assets being seized. If you'll never travel to the UK and never have any assets, then maybe you'd be okay. (though, the risk that you might be imprisoned should you accidentally find yourself in the UK down the road isn't great either...).

Some of my co-defendants have raised the point that they'd worry about their future employability as a result of being in contempt, as hiring managers aren't always the most nuanced thinkers about such things.

I personally never plan to visit the UK again but the risk of bankrupting myself and my family left where I felt I couldn't ignore this.

Ironically, the tradeoff is particularly ugly when the case is the most baseless: If the claim against you was solid-- you could ignore it and lose by default, knowing that you were likely going to lose regardless. In our case it's unthinkable that we won't ultimately win, short of outright bribery/corruption. If we failed to defend it, then we were at risk of later having local courts saying "welp this is obviously unjust but procedurally our hands are tied".