I don't think this was in service😕. on 7/12/19 several changes were made,with three cutbacks (routes 140,266 and 391) one extension (route 440) and three new bus routes, with the x140 being one of them.
It runs from Harrow bus station to Heathrow Central to facilitate the cutback of route 140,which now runs to Hayes and Harlington. don't forget to smash a like, hit subscribe and turn on notificaitons if your new.