Transferring from LHR to LGW (or vice-versa) is a major hassle. We budgeted 4 hours and it was barely enough. An hour or so to exit LHR and catch the bus, 1.5hrs for the bus journey, and more than an hour to check-in at BA/LGW (very long lines; you might have better luck checking in at BA/LHR). We were quite frazzled by the time we boarded our flight. I would definitely not do it again. If you have the option of flying out of the same airport but the cost is a little more (or the wait is a little longer), I would advise you to take that option.