No what he is gettting at that there is a full on ban in London against, BB guns, Paintball, Airsoft and Firearm discharge within the City Limits, provided the discharge is outdoors.
You may get your gear seized, it would all depend on the state of mind of the Police Constable you deal with.
So you might be able to shoot it off in your backyard with a "gentle" warning, or you might get the full tactical warning.
Either way only shoot in an area that is unviewable to the public (indoors) or out of the City Limits. Again you will need Land Owner permission.
6 Section, 2 Forward Observation Unit Airborne, Royal Artillery
Level 2 Certified BA Sniper
Sniper Instructor (Fieldcraft)