I would think the lobby to go back to LGW is that many local folk want to go on their holidays with package tour companies where LHR does not offer choice to that many
Not exactly the point of a PSO...
Regulation 1008/2008 says:
may impose a public service obligation in respect of scheduled air services between an airport in the Community and an airport serving a peripheral or development region in its territory or on a thin route to any airport on its territory any such route being considered vital for the economic and social development of the region which the airport serves.