The question really is what routes can generate traffic. LHR generated reasonable loads when the timings were right, but no-one is going to get slots at LHR. Gatwick would be the next favourite. I always thought that when bmibaby did Gatwick, that was the excuse for bmi to pull out and increase the frequency to Gatwick. It would have been a nuisance, but better than nothing, which is what we have now.
Can't see any new start generating passenger numbers.
Maybe someone like Air South West to Bristol could do something, there are a lot of potential connections there and it gives access to the South West.
Stansted would be a possibility, access to Kent, M25 and other connections. Someone like Ryanair could probably do it and make it work if the timings were right.
It is business travellers that generate the traffic and are prepared to pay for it. For years I paid bmi full fare for the convenience of flexibility and not having to sit in a car all day.
Maybe an air taxi service using VLJ aircraft might work, if the fares could be kept to business levels - £3-400 to somewhere like Northolt. They cost around £2 million have 4-6 seats, don't know what operating costs would be.