Under the Violent Crimes Reduction Act of 2004 (or it might be 2005, I can't remember) it is an offence for anyone to be in possession of a firearm or imitation firearm in a public place, without good reason unless said firearm is in a condition which prevents it from being fired (i.e. unloaded and in a case or slip). Breaching this regulation carries the possibility of a mandatory 5 years imprisonment.
The VCRA includes air guns and airsoft ("BB") guns in it's definition of firearms.
Also, if IIRC it is illegal for anyone under the age of 18 to be in possession of any air-powered firearm at any time unless on private land.
It is an offence to discharge an air rifle within 50 feet of a public right of way, if in so doing you cause a nuisance or danger to members of the public.
Under the terms of the VCRA, what happened to you was a firearms offence. The police should have been far more proactive in responding to it, and should have sent a firearms response team to investigate.