I’ve just asked a friend who knows a bit about the JJ helmets and armour. And he said this.
yes they were fibreglass, I’ve got a cast identical to that one, but it was made for me in sections so that I could use them as vac moulds. The edges around the eyes, mouth and vocoder are all pretty sharp.
Joiner had a sci-fi shop in Romford market,
One of these sold with its vac formed suit at auction a couple of years ago (at Profiles I think).
Joiner also made other fibreglass promo helmets that look a bit softer, more like the real plastic helmets. Not sure why they looked a bit different, probably moulded from a differnt source.
JJ used them at his small conventions,