Don't ignore the 50 foot rule at all.
You should not fire the gun from within 50ft of the centre of any road etc.
If you do and you even "alarm" someone - then you are committing and offence as Weevie says.
There is an excellent web site here
<Click>
It contains the following snippet :-
It is also an offence to discharge a firearm, including an airgun, within 50 feet of the centre of any highway. This includes roads, bridle-paths or public footpaths. The offence is complete if a member of the public is injured, endangered or even just alarmed by the incident.
So - you can either follow the advice of the Met Police or you can ignore it as Weevie says.