I've done a little research (i.e. looked on Wikipedia) and it seems the American Northeast was subsumed into Delta Airlines by 1972. Theoretically, the way should then have been clear for BA Regional Division to use the Northeast callsign once Bealine had proved unsatisfactory in the mid-1970s.
As for Jetset, it was arsey with delusions of grandeur but less of a mouthful than Air 2000.