LFT, you haven't read the question above. The proposition was that if the EK went to PIK to refuel and later fly on to GLA with passengers remaining on board, why was that not equally possible at NCL?
You've also ignored the fact that it's not a pick-and-mix for individual passengers as to whether they get off at a diversion statement or remain on board. Given the need to remove bags and re-screen pax arriving from a non-EU destination, you either all stay on or all get off - no half-way house which I could read into your statement that you believed may be possible.
Judging by the weather conditions in Scotland on Sunday, I would be surprised if a PIK to GLA centre journey was any easier.
And there is a further factor that few seem to have picked up here. The aircraft's next departure ex NCL was delayed due tech. None of us here know whether this was a factor, but it is quite possible that the tech issue was evident to the crew on the inbound DXB-GLA sector and would have prevented subsequent dispatch of the aircraft on a further sector (from anywhere to anywhere).
As a Captain, dumping a 777 in a non-dispatch condition at an airport where you have no engineering support and leaving Ops to sort out the resulting mess would be a sure-fire invite to tea without biscuits with the Fleet Manager, particularly when you have another alternate with engineering support available only a few minutes' flying further away. That alternate [Newcastle] is one with which you probably are familiar, have all the briefing info you need to hand, know there is RFFS and ground handling capable of taking your aircraft and you haven't got long to piss about finding out whether any or all of that exists at Prestwick given your likely fuel state. You take a decision against a ticking clock - and sadly (if the EK captain happens to be reading this) then sit back and watch the armchair experts who have never had to take responsibility for the safety of 350 folk behind them carp and criticise about why you didn't favour their local airport instead. Based on the limited information available, I'd say it was a solid and sensible decision which would have the support of any sane Flight Ops and Ops management.
Face it - PIK is not ever likely to be the primary alternate ex GLA or EDI for most carriers apart from Ryanair (who are established there) and maybe the occasional Twin Otter.