Can't speak for the police in Lancashire, but its a load of B.S. from my experience.
Never been stopped and had to wait for an armed response unit.
As for telling them its there, if you get out dressed in wellies and camo, they may well ask what you've been doing. If you tell them you've been shooting, they'll know its there and if they want to do some more checks, i'm sure they will. I've still never had anyone call for an armed response unit for when i've been stopped.