Brian
Unfortunately there is no way that it is possible, at this distance in time, to put a specific geographic location on the exact point where it became negligent for the crew of the Chinook to continue at low level on their pre-planned route instead of climbing to Safety Altitude. The only qualified Met Observer in the vicinity was the Lighthouse Keeper and he passed his observations to the BOI.
What is not in doubt is that the poor weather conditions were forecast and the crew should have been prepared for the probability of IMC conditions in the vicinity of the Mull.