This may be too much like splitting hairs. But I’m concerned that everyone should be reassured. and Michael’s post may easily cause unnecessary anxiety.
It wouldn’t be valid.
You cannot sign away your rights under UK law even if you wish to.
Whatever gave them the greater rights, their contract, or UK law, would apply.
If they spent some significant time in LA under USA contract it would be possible for their status to come under USA law if they agreed to it. But they’d have to become US residents, or be on an explicit c ontract to work in the USA under American law for a set period.
But they are known to have been UK residents all through.