It looks like it's going to be a super model.
One slight worry: in the Firearms Act 1968 (as amended) "…the expression “firearm” means a lethal barrelled weapon of any description from which any shot, bullet or other missile can be discharged…"
That's very wide; taking it to the point of absurdity, it might cover a tube which happened to have one end closed off and a small hole through the side, because if you were so minded, you could stuff powder and ball down it and detonate the powder via the hole. The Act doesn't say "commercially available shot, bullet or other missile".
If the model is realistic to the extent that it is not de-activated in any way, maybe you should have a firearms certificate for it. I appreciate that you will have no live ammunition, and no ammo will be commercially available, but the Act says you commit an offence if you manufacture or possess a firearm, whether or not you have ammo to go with it. Indeed, it would not be too difficult to make up miniature shells from items readily available like the contents of fireworks, starting pistol blanks, lead and brass (though doing so would constitute a separate offence).
Andy.