Sycamore and Robin,
Go back and read the many other entries on Weather and Altimeter.
The TAF is a forecast made several hours before. The Macrihanish actual shows the forecast was not widely off the mark.
In other words the crew made a perfectly legitimate and legal decision to attempt the flight.
The lighthouse observers and Mr Holbrooks reports are more relevant.
Again, several thousand entries ago, you will find that it was shown that the sub scale setting on the altimeter tied in with both LFP / Regional QNH, and LZ QFE.
Take your pick.