I suspect those who have swallowed the BackHeathrow line have done so based on the simple expansion of the robust model that operates "as at today" with BA a "UK" airline continuing to shuttle pax and provide service to various UK domestic points !
Actually no, why do you assume everyone is so eager to use BA?
The long haul portion of the flight will be chosen by airline with the initial departure being from a local airport. I doubt anyone will care who provides the shuttle down to LHR and different airlines or alliances may use different carriers.
Heathrow will be sinking under the weight of new debt so will pass that on, they are already talking about slashing wages by 30% and renogotiating contracts, are we seriously suggesting an airline like FlyBe who have recently pulled out of Gatwick and who have witnessed the demise of Virgin would contemplate taking on BA routes?
Heathrow is currently being unfairly held back by the inability of the politicians to make the decision.
As for Flybe not taking on the routes, why not? Again, just because BA are providing the domestic routes today does not mean it will stay that way. It will also not be seen as competition as such if BA are feeding their own flights. They may be happy to let somebody else feed people onto the long haul competition. The third runway will change things and open up this market to others. If Flybe don't want it then somebody will.