There weren't that many cars made for GTA London, but those that were have been put in with theGTA cars, simply because they share the same format (except for the doors being on the other side!). Just search for London to find those that exist.
There were some cars made for GTA London: 1969 and GTA London: 1961 which weren't included in the final game. Ray Larabie, the creator of these fine vehicles and previously Art Director at Rockstar Games Toronto kindly released these to the public. You simply won't find higher quality user cars for GTA London. Many thanks again, Ray.