SFFP
Your 7279
You are absolutely correct that a forecast is a "Best Guess".
That "Best Guess" would have precluded flight in accordance with VFR in the vicinity of the Mull Of Kintyre; according to the Findings of the BOI. The Pilots should therefore have planned to transit the Mull at or above Safety Altitude and should only have descended to low level flight again once they were unequivocally certain that they could continue the flight under VFR conditions. This they did not do.
9 witnesses on the Mull gave evidence in respect of the actual weather at the time of the crash to the BOI. All of those witnesses heard or felt the crash -therefore their recollections are specifically linked to the time of the crash. Their evidence shows that the actual weather at the time of the crash was considerably worsethan the forecast weather and therefore precluded flight in accordance with VFR at that time.
These witnesses were some hundreds of feet above sea level and thus above the cloud base. The Yachtsman was, of course, at sea level some 2 NM from the Mull with a visibility of "1 NM limited by haze" and would thus have a different perspective from those in the immediate vicinity of the Mull. Moreover, he was not aware of the tragedy until some considerable time later. His recollections will not, therefore, necessarily be specifically linked to the crash; because he was unaware of such an occurrence until much later.
Failure to comply with the basic rules in respect of VFR flight at low level when faced with poor en route weather in the vicinity of high ground resulted in this tragedy.