Unless you live airside at Heathrow and your destination is the Edinburgh duty free shop, I sincerely doubt your journey will take 1.5 hours total. More realistic timings allowing for 30mins travel either end and arriving an hour early for your flight bring it up to 3.5h - the fastest current London-Edinburgh expresses are just over 4 hours.
At those timings, high speed rail starts to look quite attractive, considering it's much easier to work and relax on a train and you don't have to waste time shuffling through the airport and waiting in queues.