If there is such a good case for P2P flights from regionals then it would be happening already. It's not only BA that flies out of LHR. There will be regional connections down to LHR with an easy connection onto many long haul services.
There seems to be a lot of wishful thinking going on here, this is an aviation forum and therefore people would like to see more flights by larger aircraft from their local airport but if the numbers are not there then it won't happen. There will never be a direct flight from Durham to Los Angeles but by using a shuttle to LHR this becomes possible. LHR expansion will put a stop to all this wishful thinking and people know this which is why those in the north who are not affected by noise/pollution issues are so against it.
The fact remains though that if so many people are flying from somewhere like MAN to a certain destination and having to connect via LHR then another airline will start direct flights in competition. Where there is a viable business case then it will and should happen. LHR expansion will open up many destinations from the regions, it is largely paid for by private investment, much of the public money being spent would be spent anyway in the same area. I really do not understand why those away from the SE are so set against it other than the usual anti London drivel.