If it was a real emergency....I think they would have.
Kef’s usual group of planes to land are all medium sized aircraft and while the runway is more than long enough....getting cargo or passengers off might not be that simple. I also think it’s only 2-hour flight back to London....so maybe they were thinking of their ‘team’ being there and able to figure out the cause rather than flying a bunch of folks over to Kef.
Does Kef have medical response teams capable of handling hundreds of possibly sick passengers?
If it was something like bad air in the plane. The pilot and co pilot could have been the next ones to fall out.