I assumed it was an English bike all along. Searched frantically for it as a Triumph, BSA or Norton and others, but could never find it. One of the grainy photos you posted looked like it might be a four cylinder or a three cylinder, ala Trident with split center exhaust.
Anyway it is a 1983 Triumph Phoenix with a TWIN cylinder. Apparently with four exhaust ports. Variously a 900cc or 750cc capacity. A prototype water-cooled, 900cc bike called the Phoenix (the engine was also known as the 'Diana') is designed in early 1983 (a water-cooled, twin OHC vertical twin). I have a little info from book Triumph Bonneville Year by Year book author Paul Hazeldine. There is a pic very similar of the donk from the other side and a darker pic of the mock up at the show taken from a front view angle, hard to see really. The text reads something like ...
The proposed Phoenix first mentioned late 1982 is a water cooled DOHC twin with 4 valves per cylinder chamber. It featured at the Birmingham Show 1983 as a complete bike mock up. It was placed in a remote area making close inspection difficult. It had four exhaust pipes, only two silencers, the engine a stressed part of the frame with a monoshock rear suspension, cast alloy wheels and disc brakes. They were up with the play really, what could have been.
The show display was in March despite the fact they had ceased production in January, and in August the company as history tells, went bung, the phantom bike was never made.
There is also a better black and white pic in "T140 and Derivatives" by Roy Bacon of the Phoenix 900 which says the engine was coded "Diana", now Roy is a Trumpy, Beeza, and Norty guru writing many books on these, he should know.
There's also some footage of the above engine actually running, so it got that far in development, but the man in the picture at the top of the post, the owner of the only bike built, mentioned that it's a wood and clay mock-up, probably the reason it was displayed on the roof of Triumph's stand at the bike show in the film.
http://www.vintagebike.co.uk/wp-content/uploads/gallery/triumph/1983-triumph-phoenix-614x570.jpghttp://www.fedrotriple.it/agg_2008_03/1983-liquid-cooled-triumph-diana.jpghttp://www.fedrotriple.it/agg_2010_04/1983-phoenix-2.jpghttp://www.fedrotriple.it/agg_2010_04/1983-phoenix-1.jpghttp://www.fedrotriple.it/agg_2010_08/1983-triumph-phoneix-show.jpg