Exactly. i read through the law a couple years ago when I was thinking of relocating to the UK and it doesn't matter at all what it's made of. The important thing is what is LOOKS like. As the law calls it a "realistic imitation firearm" can be deemed as such if you need to do a close inspection to tell whether or not it's a real firearm. Honestly, if you want to be hard about it, the PKD is borderline in that regard. Anyone that knows a little about guns will see the ORANGE grips and general fatness of it and understand that it's not a real working firearm, but on the other hand, you can take a black piece of pipe and duct tape it to a piece of wood and an old lady with so-so eyesight will think it's a weapon.
If it were me, I'd strongly point out to it having ORANGE grips and being much "fatter" than a real firearm. Also mention the LED LIGHTS on the sides and point out that it's from a "futuristic science fiction film" or something. If they bring up the fact that it has the STEYR logo on the side (i.e. a real weapons manufacturer) point out that prop makers often use "scrapped parts cobbled together" when making sci-fi weapons.
Another idea might be to send them a couple real sci-fi-looking posters from Blade Runner, showing off that it isn't from anything "real". Note however, that since Tomenosuke likely don't have any license to repro Steyr or Bulldog parts, there IS a chance (however slim) that customs might start ANOTHER fight about the "copyright nature" of the prop. (On the other hand, you could then point to the whole Stormtrooper bucket conflict where AA IS allowed to make unlicensed copies in the UK...)
I mentioned a few pages up as well that if you read the exact wording of the VCRA law, it's rather unclear whether or not a replica in KIT form falls under the law or not. Even IF a PKD ends up being regarded as a RIF, it might not actually be so in KIT form. For all they know, you were never even going to BUILD it... to preserve its value.
Another thing I also mentioned earlier- since this is a KIT, customs also don't know if you were planning on painting a large part of it in a BRIGHT COLOR- something that will also negate its status as a RIF.
Finally... do you have a lot of replicas? Do you have them photographed online anywhere, such as yourprops.com?
if so, you could TRY to claim that you were obtaining it for a n online museum/gallery (something that IS a valid defense for importing a RIF...)
Hope these ideas are helpful to you but again, as Andy says... it matters NOT if it's made of plastic. What matters is what it LOOKS like. Good luck, and please keep us informed!
If all else fails... why not contact a newspaper or two... "Scifif fan gets toy ray-gun confiscated" might well cause a stirr as to what tax money is being used for...
Aren't the UK laws against any replica gun that could look like a real gun?