Well, we have seen the statement from PKPF68-77 (a forecaster with vast experience of the nuances of weather conditions in that part of the UK) having reviewed the TAF's for that evening:--- "I certainly would not have wanted to go VFR with anyone past the Mull that evening."
There you go again Caz. Making assumptions.
The BOI agreed that the conditions were ok to plan a vfr transit. There was no intention to go
past the Mull, but rather to route northwards on the seaward side of it.
Why the aircraft did not turn along the coast is the crux of the problem, and no-one knows beyond any doubt whatsoever why it didn't.
Interestingly, you defend JP with such excuses as navigation being a more visual art in those days. It still is in Support Helicopters.