'Domestic connectivity has been falling for years, mainly due to improvements in mainland rail and airlines desire to move slots to more proffitable destinations. Despite T5 opening, LHR today serves less destinations than before, and the rail improvements are due to continue long into the future.'
A comment like that could only come from someone who has obviously never tried to connect to Heathrow from the regions. There is no train connectivity apart from central London.
Speaking as someone who has been on the BA shuttle from Manchester 3 times in the last few weeks I can assure you that the majority of passengers on those flights turned left for flight connections rather than right for the exit in Terminal 5.
I should confess in ideal circumstances I would avoid connecting at Heathrow like the plague sometimes circumstances such as meeting colleagues from the south to fly on means I do. If I had to fly to Northolt then get to Heathrow then that would cease and it would become a case of I will meet you at the destination. Maybe if you didnt live on the doorstep of Heathrow you would understand why your idea simply wont work.