Within England, air transport of any sort is of marginal value. For 'most weather' GA it is even worse. There are a limited number of airfields located near any place you are likely to want to go, the vaguely economic ones have no lights, limited hours, no approaches. Some of this is driven by regulation, some by tradition, and some by an industry strategy to make money by high margin rather than high volume (Gloucester seeming to be a notable exception with lights, approaches, friendly environment, reasonable fuel, reasonable landing, no mandatory handling).
London to Birmingham is too close
London to Manchester means landing in Liverpool or at the main (and frequently expensive) international airport.
The South into Scotland mostly has 'Bend Over Here It Comes' pricing (other than Cumbernauld which is quite convenient for my work).
I use a SEP for transport to Rotterdam, Carlisle, Glasgow (Cumbernauld), Dublin (Weston), and Newcastle. All of which are more convenient to get to from South West London by GA than by any other means.
The level of landing charges and mandatory handling in some of these places is rather high for the service (Rotterdam is expensive, but I use the facilities quite extensively so it is not that bad).