Thanks Maggot,
I am aware southbound, LHR-PER is shorter, but the list of suitable alternates in the Indian Ocean is a lot shorter too, if you are chasing the Westerly they will be south of Sri Lanka, 180 mins from Jakarta then its Port Headland and PER. Then even if you have 10T-12T on arrival PER, You won't make ADL, and really, who wants to go to Learmonth or Kalgoorlie?
Ushuaia,
PER-LHR, sure, duty time will be restrictive, but i'd rather be in a hotel in Frankfurt/Rome/ than the airport terminal at Learmonth/Kalgoorlie/Port Headland and running out of duty.