I don't really understand why the group would do this.
I understand why they would want to take as many O&D passengers as possible out of LHR and that travel times from STN into central london is similar and probably less hassle. I dont think competing with easyJet to UK routes is going to go very well, unless the group can offer something more than easyJet, with DC miles, lounge access maybe a (very) light refreshment.
I call bluff on the A319s moving over from mainline to a 'rebranded' baby. BD do use these aircraft at LHR, on their European routes and the remaining domestic network. They are currently refitting them with new seats, so unless baby is being folded back into mainline, I don't know. Last plan was some more 733s and new paintwork for the rest of the baby fleet.