If I'm reading some of the answers posted regarding De-acts not needing certificates, correctly.............umm, why do they ALL come with said piece of paper, then ??
I seriously would not want to be stopped, whilst carrying said de-act without cert,........at best I'm thinking you'd have a good talking to by the powers that be and don't forget, just because we know our stuff re wwii kit/guns/uniforms, etc, the average policeman doesn't,......and a guns a gun;.........if armed response was called,...a whole deep pile of poo is tipped up.
Also, a lot of events, now are asking to see de-ac certs of those displaying,..and I've even heard of an event organiser refusing to allow those without certs to exhibit.
Regarding Imitation firearms, (RIF's) whilst its true you don't need a certificate to own one, most dealers/sellers will want to know which group you belong to, (MVT and IMPS are recognised groups)and also to know you've public liability insurance, (which, if your showing your kit, off vehicle, makes sense to have anyways.)
Shooters Rights and AFRA are also people to talk to in respect of this.
As you say, its certainly a question that does get asked quite a few times,.......so its best to get it right. :-)
All the best.
Andy