I spent an enjoyable few hours at the London Model Railroad Group. They have been around since 1938 and moved to their present site in 1970. They operate a very large 2 rail O-Guage layout in a room that is 32 by 12 meters.
Most of the buildings have been scratch built and the layout is controlled via block sections, which allows 4 mainline trains operating at once. There are some fantastic rolling stock and engines, many of them scratch built as well. If you are in the London area it is definitely worth a visit.