TBH I'm not trying to state the 'worst case' here at all. Just trying to inject a little clarity and to help you, as the original questioner, understand what your position in law might be. I know some folk don't want to hear this stuff - or prefer to believe an interpretation of the law which better suits their own views/actions - but that's their risk. And coming on to a public forum and potentially incriminating yourself is probably not the best advised course of action!
Remember, the risk is considerable. This is not just 'slap on the wrist and we confiscate your RIF' territory!
The law is actually pretty clear. If folk choose to be 'wilfully ignorant', that's up to them. But it won't avail them in court if they do get pulled up by the authorities and may, in fact, serve to make their situation worse.
Certainly mitigation of risk is important. But that requires a demonstrably intelligent, engaged, thoughtful approach. The Act sets out what your defences are.
My (unprofessional and at your own risk) suggestion would be that if you are a private individual and intend to mount, or have already mounted, a RIF in or on your MV then you should join a recognised and respected historical reenactment group that holds PLI and is relevant to the age of your vehicle. And it would be advisable for you to actually use your MV in some kind of reenactment and to have proof of having done so. Then I think you would have a decent defence.
The fact that your RIF is an imitation of a weapon that might have to be crew-served is NOT a defence!