How are they going to seek the damages given the defendants probably don't own any property in the UK?
That said, other countries will frequently enforce foreign judgements. In this case, the specific performance they're asking for would likely be unenforceable in the US (and useless regardless) -- but damages might well be enforceable.
The most horrible part is that _they are allowed to drop their case anytime_ in the UK civil court. In previous cases when the accused has run out of money, the defendants have to pay their costs [1] to keep the trial running - with zero chance of being able to recoup costs because the accused will be bankrupt anyway. The UK state should not make private individuals fund the creation of novel case law; it's absolutely stupid.
[1] https://www.dpsa.uk/wp-content/uploads/2021/11/Malkiewicz-v-...
If you read the appeals decision (and the decision that granted the permission to appeal)-- that seems to very much be the motivation here too: Case law on this subject would be intellectually interesting and potentially important in the future, so lets leave these foreign defendants on the hook to help establish it.
Beyond the ethical problems with that position to begin with, this is a terrible case to try to establish anything because it's so factually conflated that all the resources are going to go to sorting through the plaintiffs lies. The defendants don't have any particular interest in the UK establishing good law except to the extent that bad law in the UK may have international ramifications. (and even there not much-- defendants aren't businesses, they're volunteer developers many (most?) of whom have already quit participating)
To extend the parent question, why can't the people accused, not being in the UK, simply ignore this?
There are good reasons to expect that in this case they would be unenforceable in the US, but that isn't a guarantee-- and if they are you don't get to go back and fight the cause you lost by default and would have expected to win on the merits. So you're risking a kafkaesque situation where everyone is saying "yep, it's wrong and unfair, now hand over all your assets. thems the rules".
It's also the case that ignoring a judgement in the UK can get you found in contempt and subject to prison should you happen to find yourself (perhaps accidentally) in the UK in the future. Personally I have no problem never going anywhere near the UK in the future, and though "outlaw in the UK" has a nice ring to it, this does add to the incentives.
The person who runs the bitcoin website was sued by the same con for distributing the bitcoin whitepaper, and defaulted because the UK wouldn't allow him to defend himself without divulging his identity. He took the whitepaper down from the UK under threat of arrest (if they ever find him) and now facing hundreds of thousands of pounds of the opposing side's legal fees under threat of potentially losing the domain name (and/or prison, again if they can find him). There really is no silver bullet.