Just to be clear, even in North London areas alternate between on the tube or normal train lines, and there is a large tubeless area of West London between the Central Line and Piccadilly Line, and in East London either side of the Central Line. This is one of the reasons for Crossrail being built to take over the local services into Liverpool Street and Paddington to upgrade the capacity on those routes.
I would say that most of London is near a tune line. Its just parts of South London that dont. Its no surprise that the places that are not have tube stationd nearby dont have top class football teams nearby
The Broncos have averaged over 5,000 in Super League at both the Valley and the Stoop, so it is a very poor excuse to use not being on the tube as a reason for their current sub 2,000 attendances. The union club also have no problems packing out the Stoop travelling by train.