LBA-ISB is considerably shorter than some of the 757's routes which beat it by a good 350 miles. The only problem might be Jet2's high density configuration, Leeds having quite a short runway (shouldn't be too much bother for the mighty 757, but Muslims don't travel light from what I've seen at NCL) and Islamabad being hot and quite high.
Apart from that if Pakistan let them in 3/4 weekly on a 757 could work... They would probably need to buy another 757 though, 757 200ER?