Flipster
<<so its not as simple as it sounds, is it!? >>
Well it's simpler than you seem to be making it appear – the area of interest is the low hill that is the Mull which is isolated on their route by many miles before and after – and in that area icing considerations would not have precluded the option of going up to SALT – it is that simple.
The Mull is an isolated low hill that on that day, as it so often does with the prevailing wind in Summer, was generating local orographic cloud and upslope mist right on the landmass such that the Mull could be seen from a distance but topography and surface detail was masked so as to make distance off judgment very difficult visually.
If there was no need to approach it closely, it could have easily been avoided by turning away while remaining VFR over the sea – had for whatever reason they needed to go straight over it, icing was not a limitation.