Flights of around 7 hours are no problem off our runway the A380 did it to DXB. Flight time to Islammabad is a tad under 7 hours.
I think we're mixing flight time and aircraft performance. The more modern aircraft needs less runway and has greater range than the previous generation, and in most cases won't be packed to the rafters with
outbound cargo.
The fact that BHX has attracted two of the worlds most basket case airlines is being celebrated in terms of the aircraft they fly, not the markets they serve or the companies themselves. It's not impossible that someone might want to fly from China to BHX, however until LHR-China is maxed out and a mature market, I can think of no earthly reason why they would. HS2 is only going to make it easier to serve Birmingham from LHR. BHX is getting killed on long haul in exactly the same way as LGW, it's overwhelmed by being too close to a critical mass LHR.
A 787-9 requires a 3000m metres certified runway at MTOW at sea level.
I would think
MAXIMUM take off weight won't be that common as bulk of the cargo will be most likely be on any inbound UK leg. Even with every seat filled and a decent fuel load, where would you be going from BHX that would be at MTOW?