"A person commits an offence if, without lawful authority or reasonable excuse (the proof whereof lies on him) he has with him in a public place an imitation firearm."
http://www.legislation.gov.uk/ukpga/1968/27/section/19
That's the primary risk for us, not the VCRA. Reasonable excuse isn't a high bar, just try to have something
to hand showing what you're up to, whether that's a site booking, a Facebook message or event "attending", or some texts saying "Meet up for green beret SAS ranger operator training lol".
By the way, "public place" covers anywhere to which the public enjoys a general right of access, which is surprisingly broad. If it's not your land, and you can get access to it without committing trespass, then so can anyone else and it's likely to be a public place. If you are committing trespass , then you're onto a Section 20 offence, with the same defences.
tl;dr version - in this case, it is better to ask permission than to seek forgiveness.