I suspect PIA will prioritise returning to BHX, and wouldnt be shocked if given the amount of competition from LHR, they prioritise the regions ahead of London.
Thats said, the last few years has allowed the likes of Turkish, Qatar and Saudia to really start to establish their market out of the midlands with their vastly superior service and dependability. Yes there will be people willing to compromise service standards for cost and direct flights, but PIA will find it hard to win back their past market. Plus anyone not living in Islamabad and requiring onward connections, would you rather connect through Turkish, Qatar or Saudia.... or PIA?!?
I would welcome the return of PIA... but i legit dont think Turkish, Qatar or Saudia would see much threat in it!