I’d say you are spot on with your speculation. Your memory is certainly fine 
UK Supreme Court judgment here: Lucasfilm Ltd & Ors v Ainsworth & Anor [2011] UKSC 39 (27 July 2011)
TL:DR - the storm trooper helmet is not covered by UK copyright (it’s not a sculpture). It is covered by design right (it’s a utilitarian object produced for an industrial purpose, not a piece of art).
UK law allows manufacture of copies of objects protected by design right 15 years after the item is first put into production so Mr Ainsworth’s helmet sales were fine.
All of that of course only applies under UK law.
Mr Ainsworth first got sued in California and the court found in Lucasfilm’s favour (judgment in default) and awarded damages to the tune of $20 million.
That judgment still hangs over Mr Ainsworth. The UK courts declined to enforce it on the basis that there was not enough connecting Mr Ainsworth to the US.
Reasoning for that set out here for those who really want to know:
Lucasfilm Ltd & Ors v Ainsworth & Anor [2009] EWCA Civ 1328 (16 December 2009) (from para 187 onwards) Go on, you know you want to really…
I would be very surprised if greater UK recognition of US judgments is not something that lots of US companies are lobbying to have included in any trade deal.
It was/is certainly part of T-TIP.
This case of course highlights the big difficulty with that. Unless you have the same laws and apply them in the same way, there will always be cases where something is perfectly legal in one jurisdiction but not in another.
If you want frictionless crossborder trade you have to harmonise rules to a sufficient extent that someone in one country can be reasonably confident that nothing they do legally in their country is going to land them with a $20 million judgment in the other.
Given the well-known eccentricity of some US judges, our confidence in your justice system is not high.
