That's different, because the copyright laws are different in the UK. There, whoever creates a character owns the character and gets royalties for their use
...
But in America, writing for a screen franchise is work-for-hire.
That's not
quite right; it's not really that the laws are different, as the owning corporation of a screen franchise
could hire authors under a situation that isn't work-for-hire, they just choose not to. Like the situation with Siegel and Shuster regarding Superman; the (successful) legal argument in the 1999 lawsuit was that Action Comics #1
wasn't under a work-for-hire contract but rather as an independent contractor, and that the work for hire relationship only started with #2.
Work-for-hire in the UK, though, is pretty much exactly the same as it is in the US as far as I can tell. It seems that the difference is the same thing; Nation wrote as an independent contractor, not under a work-for-hire contract. Either that, or the Copyright, Designs, and Patents Act, 1988 changed the situation from what it was previously when he created them. Those are the only explanations I can think of, as the CDPA 1988 spells out the work-for-hire situation pretty much the same as the way it works in the US.
(One other correction, Nation doesn't have total ownership over the Daleks the way you describe. The IP is
jointly owned by the BBC and Terry Nation, so any usage requires the consent of
both parties.)
Edit: Checking more into it, it looks like the BBC just actually
preferred to give their writers shared ownership back then; they explicitly went out of their way to hire writers as independent contractors with a shared copyright rather than under work-for-hire contracts? So it was more of a kinder corporate culture thing. By offering shared ownership, they allowed their writers more agency and more chances to use their creations elsewhere while also ensuring that IP they were involved with couldn't be used willy-nilly.