What's the point of the runway extension?
Realistically how many flights per day would use it versus the expense and hassle to build it?
Rrealistically, none in my opinion.
BHX already handles the B773, and has handled the A388. The longest flights are to Pakistan/dubai, and frankly, thats as far as the majority of BHX flights will go.
EK is successfully covering the middle east/far east and Australasia, Mahan is covering the middle east and going west, CO covers the USA segments.
With this in mind, few carriers beyond Dubai will look at BHX and BHX seems to have a strange problem of not being able to support 2 daily flights to the USA (CO tried double daily, US/CO tried 2 destinations, CO/AA tried 2 destinations, and all have always whittled down to 1 daily again)
Maybe better to concentrate on terminal upgrades and more parking stands? but Like I say, this is just my opinion.
I know some will say 'well, its ok for MAN with its 2 runways', but this is a problem in itself, its a financial burden that cant pay its way as its closed most of the day thanks to noise/traffic quotas, BHX should learn from this and think again.