Will quickly point out the Jury's Inn near Hatton Cross. Though £79 is beginning to push it for the class of hotel (you do get 1,000 Diamond Club miles if that is your sort of thing). The first tube of the morning from either Hatton Cross or Hounslow Central will get you into Heathrow T5 for 5:14, still plenty of time for a GF visit and should save you the cost of a taxi.