When I first started with BA 2001, majority of long haul to Africa, USA and Middle East was from Gatwick North. But gradually moving to LHR T4 and T1.
For staff travel I prefer going through Gatwick, the staff are very nice and helpful, where as if you go from LHR T4, you have to que up standing in Staff Travel where the staff couldn't really careless about you. Its really crowded in LHR T4 now than before.
A lot of Staff and our Customers prefer Gatwick North, for example pax comming of flights from the USA to Gatwick then onto Lagos, now have to go all the way to Heathrow.
The worst is LHR T1, well domestic side anyway, it is very limited for customers needs, but soon Long Haul flights like Tokyo, South Africa will move from T4 to T1.