I think it has to do with where the transmitter is located (and no I don't know exactly where they are). Coming from the south you can pick up London main and south at pretty much the same time. London north comes much later. Coming from the north (I'm guessing all Scottish traffic and a lot of transatlantic stuff) you'll pick up north well before south and main.
So by my own flawed logic, main and south will be very close to each other so if lgw is on main or south makes no difference. North is located much further north thus giving Atlantic traffic a much quicker heads up.
Continuing on that theme, more a/c flying to lgw will be charter (traditionally) and so less likely to be fitted with acars, whereas heathrow stuff will be legacy and do have acars and can pick up heathrow wx online so less likely to need volmet. I really am trying to think logically here and so could be completely wrong. I'm taking 2+2 and getting 73