This page:
http://www.publicdomaintreasurehunte...ks-in-the-u-k/
...seems to suggest that the rule of the shorter term probably doesn't apply in most cases because it didn't apply between 1957 and 1996 and wasn't retrospective.
In this particular case the works are not US in origin (I don't think).
Quexos, if I choose to break this law, I'd at least like to know that is what I'm doing, and I'd definitely prefer not to pay someone else to break it.