If Flybe are not interested - and clearly they are not - then I would think it extremely unlikely that anyone would or could consider MME-London. The economics of operating smaller aircraft into the London area airports are becoming harder and harder to sustain. Gatwick is progressively ramping up its charges with the management's stated intention of getting rid of aircraft with below 150 seats from there in the next five years. It's following Heathrow's lead, and the ramp-up in LHR fees is what led to MME-LHR and LBA-LHR being axed in the first place.
Even before you consider issues about slot availability (Heathrow - not a hope, Gatwick - maybe, London City - maybe) then you have a major economic problem. You need peak-time arrivals and departures for domestic routes where travel times are stacked around the morning and evening peak hours.
For an aircraft like a Dash 8-300 or Fokker 50 carrying about 40 passengers each way in and out (generous), the airport landing and passenger charges are:
LHR £1,396 but subject to a minimum of 18 pax on your aircraft
LGW £1,188 but subject to a minimum of 27 pax on your aircraft
LCY £1,772 but subject to a minimum of 37 pax on your aircraft.
Assuming an average £90 one-way fare including airport charges but excluding APD (which is quite a generous assumption) then you need to carry 16 pax at LHR, 13 at LGW and 20 at LCY on the roundtrip just to pay the airport fees at the London airport. At LCY, that's a quarter of your income gone just to pay their airport fees, before you pay anything towards fuel, Eurocontrol, ground handling, aircraft and crew fixed costs etc.
There may well be demand between TeesSide and London. I wouldn't dispute that. The problem is that no airline can serve that demand profitably because the charges at the preferred London termini are so high. Yes, LTN and STN are cheaper but they are also less attractive to the customer bound for central London.
And before anyone asks how LBA has a Gatwick service and MME doesn't, please refer to the earlier comments about Gatwick's stated intention to further hike its charges to force small aircraft out. LBA was launched early last year and the new owners took over LGW late last year. It is hardly wise to launch more services with a threat like that hanging over you. It's a key part of the reason why the new bunch of w***ers running Gatwick will have unused runway slots at peak times this summer for the first time in living memory.