If you're doing a turnaround, on a seperate ticket (e.g. LHR - KHI, then buying this cheapo fare), wouldn't it be extremely risky to be at the mercy of the airline agent at heathrow explaining why you don't need a visa? Since you're on a seperate ticket, how easy/hard is it to explain to them this?
I remember recently reading a travel blogger doing the same thing at SGN airport (Vietnam) where he was denied boarding BKK-SGN to catch the cheap qatar fare out of sgn.