Simply expand Gatwick and let that solve the problem in the south east. No need for an extra runway at Heathrow. Why have flights to Murcia, Alicante, Mykonos, santorini and so on from Heathrow taking up slots that could be used for business routes connecting the uk to the world? You know the ones that we are apparently lacking that only a third runway could provide. Keep the leisure at Gatwick, which after all it is what it is good at as you say.
It's not that simple.
The leisure routes you speak of are BA short haul at weekends when the A320 series fleet would otherwise be less busy on days when traveling on business drops. So for two days a week you could cram in a handful of long haul flights but that barely addresses the capacity issue at LHR. I agree expand LGW by all means, nothing is stopping the long haul airlines of the world using it today.
You think a second runway at LGW means our hub capacity issue will be fixed? no, because hub capacity is at LHR and that's almost maxed out. You're talking about point to point traffic, that's a similar but not identical issue and you should not mix the two together. LHR only has the connectivity is has because of the short haul, long haul balance, and that is in danger of withering. That's the danger that was identified and will be addressed by LHR expansion.