The Safe Alt around Ben Nevis is approx 6500ft (near Corran), which was above the Scottish Transition Level of around 5000ft. There was a low QNH/RPS of approx 989 mb which would have equated to a Safe FL of 73 and therefore, a Safe Quadrantal FL (tracking to the NE) of FL 90. Of course, they may have got a radar service at FL72 or even initially 6500ft, from Scottish Mil and could have remained there. Also, it is possible that they may have got 'VMC above' (cloud tops) at at a lower altitude. Sadly, airmanship dictated that they could not have depended on these possibilities and would have had to plan for the worst case; somewhere far above 2400ft and well into the forbidden icing band.
I do believe that D and W (and their accolytes) insist that 2400ft was a legal SA over the Mull - absolute poppycock, as anyone with an ounce of airmanship will tell you!