Jayteeto
Quite right - a good starting point for discussion of the weather - and the Antrim hills in the area you were in got as high as 1500ft ish, didn't they? - so imagine being at say, 500 - typical conditions in that area, clear for a long way if you are low.
AND so typical, given that the air at sea level was damn near saturated that day along with that strong prevailing wind blowing off the sea onto the Mull, you could count/absolutely guarantee/ on there being generated the ground hugging mist I have described.
This would have followed the slope up until it merged with the orographic cloud - a grey slope running up into a grey ceiling.
You could see the landmass - with occassional blotches of ground but otherwise ground detail obscurred - bad for judging distance.
It's only a day's drive for you lot in the UK - go up there for a few days in the summer and picnic there in the late afternoon when you have a southerly ish forecast of moderate strength (quite common) - and wait - better, go fishing just off the shore in a little boat. That is if you really want to get a flavour of the conditions. Oops, you've all missed this summer? - well number 14 will be around next year.
Understand the conditions as a starting point - then work out what could have happened.
There is no evidence of a/c fault - there is evidence of their having some extra task.