Gotta be careful disparaging LHR on here, I don't know if it's the same MoDs, but the level of sycopanthy towards LHR was extraordinary at one stage.
I don't see the advantage in ringfencing slots for marginal routes to LHR. Given the cost of the project, LHR will want as many available for lucrative long hauls as possible so they can try and pay for the project without jacking up prices. There is also the issue of HS2 coming online.
There is also the issue of the UK putting all its eggs in the London basket again, which is not foot for the country as a whole.