This week, I flew MME-LHR on a 49 seat E145. It was just me and 5 other pax, or 12% load factor. Fares about £90 oneway about 2 days in advance.
Maybe I picked an offpeak date, but this suggests to me the route has some serious issues.
Darlington-London trains are excellent. Even Eaglescliffe-London direct trains are OK. Maybe having AMS as the sole hub connection is for the best.