Yep, deactivated firearms were a loophole in British law many years ago. Some criminals bought them legally and then reactivated them (illegally) but that loophole was closed a long time ago. Many types of air and BB guns were also made illegal and even toy guns are illegal unless they're painted bright yellow or orange. Basically anything which the common idiot could mistake for a real gun is illegal!
"Section 16a Firearms Act 1968:
It is an offence for a person to have in his possession any firearm or imitation firearm with intent--
1. by means thereof to cause, or
2. to enable another person by means thereof to cause, any person to believe that unlawful violence will be used against him or another person."
In other words, even if you duct taped two carrots together to look vaguely gun shaped and used that to film with, if someone watching from a distance
believes it to be a real scenario, then you have broken the law. I very much doubt the CPS (Crown Prosecution Service) would actually prosecute a case like this but in theory you could face 10 years in prison for "possession of a carrot with intent"!
You mentioned military grade weapons. Well, if in theory you can be jailed in Britain for possession of a duct taped carrot, then a real military grade weapon is a bit of a no no!
In Britain to have a military grade weapon you need a Section 5 Firearms Certificate, which is impossible to obtain as an ordinary member of the public. Every Section 5 certificate has to be personally signed by the Home Secretary (Secretary of State) and they're mostly only given to government approved firearms dealers and even then, only after years of training and very lengthy medical and personal history checks.
In some respects importing or owning a replica is actually more difficult than owning a real firearm in Britain because to apply for a standard firearms certificate (not a section 5 certificate) the first stage is to prove a professional requirement for a specific firearm, say if one is a Veterinary or a Farmer with a pest problem, but a replica is essentially useless and it's difficult to prove a professional requirement for something which is useless!
Given that the Queen of England is also the Queen of New Zealand (and the Queen of Australia) I would assume there are certain similarities in our laws and from what you've said, this seems to be the case. A standard gun certificate is also issued by the regional police in Britain and the law includes wording such as "reasonable need". The word "reasonable" is obviously open to interpretation, which can result in slight variations between different regional police forces. At your local police in New Zealand you might find that an officially deactivated weapon is no more difficult to get permission for than a replica and would probably look more convincing when filming, so it's worth at least asking them about it.
From a British perspective (and possibly from a New Zealand and Australian perspective too), phrasing a question like - "You can't just say, "I'm buying this for use in a movie." and off you go?" - seems a bit weird because our starting assumption would be that it's basically illegal to posses, own or import real or replica guns and that any legal way around something that's basically illegal either doesn't exist or is likely to require significant bureaucracy.
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