There's a restriction that no gatwick departure may be vectored over Horley (small dotted area displayed on radars) regardless of height, but no such restriction applies to Heathrow departures, for example westerly detling departures leaving Epsom on approx 130degrees, level at, or even climbing to 6000'. So a Far East bound Heathrow heavy at perhaps 5000'(if the controller is feeling brave) is fine, but a light Manchester bound shuttle out of FL100 is not. Go figure...