Caz,
We have been here several times before. The Board were using a forecast made several months before and about eight hours old when the crew arrived off the Mull.
Experienced or not the BOI were making an "educated guess".
Icing never occurs where it is expected. Always somewhere else.
How do I know? I spent five years with Met Research Flight flying in and out of cloud. They were always predicting cloud composition and they were often wrong. Even with almost real time information.
Even flying Snoopy through the cloud five minutes in front of them would not guarantee them safety from heavy icing.
No one knows the "actual" conditions at the time.
Can we move on please.