Seeming as Heathrow will be applying to increase the flight cap anyway to 720,000 to make use of the third runway, why would "asking" for 25,000 extra flights to be allowed from 2021 be a problem?
Both Manchester and Birmingham airports have recently been putting out claims about how expansion in the SE (particularly at Heathrow) would harm their business. Birmingham seriously seems to fear losing flights to Heathrow if they get the third runway. If the only way these airports can secure flights is by constraining other airports, is there really a demand for them at those airports?