An advantage for a domestic route between Plymouth/Newquay and London City is that there is no service from these points to Heathrow, Luton or Stansted. It is the substantial services from these which has done in operations from LCY to places like Belfast.
I also hope Air SW makes something of the quick connection opportunities at LCY to link the Southwest to the major European cities like Amsterdam or Frankfurt. It takes a sight less time to transfer there than at Gatwick.
For London passengers, Gatwick seems miles from anywhere. Unless you happen to be close to Victoria it takes significantly longer to get there than many of the flights take. The attitudes and organsiation at Gatwick, and the time taken to get through the place, are also more suited to once-a-year holday travellers than to regular business users.