derelicte,
There are only so many people in the NQY catchment area, of those only a certain percentage will need to fly to other parts of UK and internationally, in the old days they were all funneled into one NQY/LHR/NQY route and the route was viable, take away that 'funneling' by offering them DUB, LGW, GLA, MAN, NCL etc, whilst withdrawing the LHR service, then one or more of those routes will fail.
I would suggest that MAN has replaced the LHR route!