There used to be a time when every high street in London had a model shop or toy shop that sold models, but that's been in decline since the internet now there are only a handful of shops left.
I would try to visit both Hannant's and the RAF Museum together they are just a couple of hundred yards away from each other, they are both in Colindale about 30 mins on the tube train from central London and about 10 mins walk from the station or a couple of stops on the bus.
The Model Zone is also good and right in the centre of London, the only downside is they do not tend to stock many Hat sets for some reason, they do have lots of Italeri and Zvevda sets and Vallejo paints.
Hannant's Unit 2, Hurricane Trading Estate, Grahame Park Way, Colindale, London NW9 5QW
phone +44 (0)208 205 6697 (past the RAF Museum in the blue and beige buildings).
RAF Museum London
Grahame Park Way
London, NW9 5LL
Phone: +44 (0)208 205 2266
The Model zone 202 High Holborn, London, WC1V 7BD
+44 (0)20 7405 6285