John Forsey Guns, Welling (near Dartford) Kent. Not exactly London but not far down the A2. Go and speak to John or Phil and see what they can get for you. You might be in there for a while if you go on a Saturday as it's a 'proper' gun shop and always busy. More shotguns than rifles really, but they will certainly try and help you out.
I wouldn't recommend the gunshop in Barnet. I can personally testify to bad service and learned of other's run-ins once i was burned.
Personally i'd take a trip out to Essex Gun. They're just out in Romford, boast a large gun room (2/3 shotgun, 1/3 rifles) but it's the willingness to help and lend sound advice that kept me returning. http://www.essexgun.com/index2.html
If travelling by car then visit Roding Armoury also. They're situated in the little village of Abridge (just off the A13). Make sure you visit the shop which is located in the carpark to the rear of the resteraunt/pub and not the little airgun shop in the main street corner.
Allocate at least 1-1.5hrs for the visit because you really will be in there for ages. The little shop front belies the masses of kit they have and the masses of knowledge they will offer. They will have every manner of firearms, round, shotgun, etc there.