If they do see the initial sculpt as a work of art then I don't see how they can say the helmets are not. The helmets are then copies of a work of art and AA's replicas would then be unauthorized copies of a work of art for which he has no right to.
That's like saying a bronze sculpture is not a work of art because it's a copy of an original clay. In fact, the original sculpture is almost less a work of art because it's an ends to a means. The final product/artwork is the helmet.
The helmets were made to be seen on screen. I would interpret that as being art on public display. The term "industrial design" implies functionality and these helmets are designed for form, not function.
I think the law was poorly interpreted.
The legal status of the original sculpts was not ruled on so it can be argued that it is or isn't sculpture depending on the intention of the sculptor at the time.
The cast bronze analogy is flawed. When applying the legal guidelines you have to look at the end product and its purpose. If you take the time and effort to cast something in bronze you can infer an intention it is meant to be a work of sculpture. It isn't decisive, but is clearly indicative.
This brings another issue discussed in the judgment; that of permanence. It was another factor which was deemed to imply an intention to create a work of art. Your bronze is cast in a strong alloy presumably so it will last a long time. The helmets were made of thin plastic, and most were discarded after the production ended as they were too beaten up to be worth saving. Indeed some shots from late in the prodoction show sets of armour with bits missing, and other bits being held together with gaffer tape.
This implies they were made as a means to create art, not art in its own right. The clay sculpture used to create your bronze is intended to be destroyed in the process. It appears to fall into the same category. From this perspective the original clay sculpt of the helmet may be the same. Thinking about it this is probably the case. If so this does not detract from it being an artistic work, but it does preclude it from being a work of art; at least in a legal sense.
It all depends on the intention at the time it was made. This can be the only logical determining factor as the answer to the question, "What is art?" is deeply subjective. To have any certainty (which is a fundamental necessity) in law you have to objectify the question as much as possible. To that end all concerns about artistic merit were expressly discarded as these are philosophical questions with no real answer; unanswerable questions have little place in a court of law.
As gonk27 and Lord Abbadon have both noted times have changed since 1976. It may well be possible to argue artistic intention in prop making in future cases as there is now a merchandising culture which simply did not exist in 1976; in fact it was Star Wars which is largely responsible for its genesis.
It may be possible to argue that the props are no longer a means to an end but have a whole sub-culture built up around them, and in making props you are not only catering to the immediate needs of the film maker, but also to those of that sub culture. This very board acts as evidence of this point, especially as it counts many of those responsible for making these props as members. Of course this argument only works for more recent films (if at all) but it is a start.
I apologise for the length of these answers, but this case facinates me on an intellectual level as well as a purely geeky one. In fact it was in researching it that I stumbled across this community.
I understand why some here are unhappy with this decision; I myself am conflicted. The problem is this is a law being interpreted into a situation it was never designed to deal with, and being made to fit as best it can. The problem is with the political situation in the UK being as it is, it will be very low on the agenda to change it.