This is a little off topic, but would it be fair to say that Northeast was essentially BEA in all but name from 1970 to 1976 ? Just a thought. I can't speak for Cambrian as I'm not that familiar with their operations.
Apparently, when BKS was using the Viscounts on the LBA-LHR route in the 1960s, the midday rotation was operated by a BEA Viscount. This is before my time but I don't remember this during the Northeast days.