Domestic connectivity could be ring fenced with x% of new slots allocated to new unserved destinations. Remember how well INV-LHR did for Dan Air even though it was a stand alone? So INV, JER, GCI, IOM could all support a LHR link but at the cost of some LGW/STN connections. It would allow connectivity to the world without the troop around the M25. It can be done, there just needs a pricing policy to support it as part of the agreement to expand LHR.
I doubt LPL, BHX or NQY could make the case but it depends how supportive any government wants to be outside the craziness of the London bubble.
Keeping those markets out of LHR serves MAN in a small way as they can be connected one stop via MAN in a way they cannot be over London.