As a bus example - TfL Buses get £374m per year subsidy (some from Central Government) - outside London funding is much lower - maybe £2/3m per county (and none from Central Government)
Why should buses in say Manchester be expected to pay their way and therefore get chopped and changed with high fares when London's network is fully subsidised and regulated with low fares*. Maybe that's why bus use is declining while London's is growing.
Because it is cheaper to subsidise the buses than it is to try and improve the road network in London. I maybe inclinded to drive into most cities off peak, however I would do everything I could not to drive within the north/south circular almost at all costs!
With the cost of motorways costing about £30million per mile (not including London land values), you don't get many miles of motorway for £374m per year and you would need a lot of miles of motorways to even get close to enough road space to cater for London traffic. You would also have to tear down large chunks of London to do so (meaning a lot more building over the rest of the UK)
It is very difficult to put a cost on public transport travel within London in such a way that it was a fair way of comparing it to the rest of the UK.