Manfod
Yes, I've also seen that chart. And it does include passengers going on indirect routings such as LBA-AMS-XXX.
As you say, SFO has much higher O&D demand than LAX. I suppose that LAX also suffers from it not really being on the way to anywhere, limiting ongoing connections. There is Hawaii and, at a push, Australasia (but lots of competition to Oz/NZ). So I wouldn't put it high on the list of potential new routes ex-MAN.
I agree with your comments about the crudeness of defining catchment on the basis of 2-hr drivetime. You really need to factor in competition. You'd have to be a serious LHR-hater (or MAN-lover) to choose to travel from, say, Warwick to MAN to pick up a once-daily (or less) service to, say, LAX, as opposed to the multiple dailies out of LHR.