Companies like BP, Esso and the so called premium retailers pay car manufacturers to recommend higher priced fuels on their cars. That tells you all you need to know about the supermarket vs premium fuel argument. All fuel is made to a minimum standard, premium fuels have additives which claim to improve performance, reduce wear etc but its all subjective.
Personally I use supermarket with an occasional premium as a treat, seems like a happy medium...
As for your trip up to Skye, a single tank of what you perceive as cheap fuel isn't going to matter a jot, even if you believe the hype.