I'd stress the "responsibly" part, you don't want anyone seeing it until you're in the safe zone. The actual consideration isn't use at the site, it's transporting it there, since public possession of an imitation firearm (realistic or otherwise) is an offence under Firearms Act 1968 Section 19.
We all need a "reasonable excuse (the proof whereof lies on [us])", i.e. booking details or at least the site info when you're on your way to or from the site with it. Don't be tempted to take it out and show off for your mates, it's just not worth the risk of getting a real one pointed at you by an irate firearms unit copper.
