So that leaves me with 182lbs (83kgs) for fuel which in my calculations is 115 litres (25 imp. gallons) - 8 gallons / hour burn give me an endurance of c. 3 hours
Maybe it's pedantry, but I never count reserve fuel in endurance calculations - I'd have said that 25 galls gives you an endurance of 2 hours, with a 1 hour reserve.
Awaiting more details of winds on the day, assume a flight time to Prestwick (for fuel) from LBA at 100knts/hour 1hr 25 mins mins point to point + 20 mins on ground at LBA + 25 mins approach at Prestwick gives a total of 2hrs 10 mins with 50 mins reserve
All sounds a bit marginal - 100kts groundspeed could well be optimistic. If you're expecting to use controlled airspace have you allowed for any possible reroutings that might be necessary?
As another Scottish-based Warrior-flyer I'd do it slightly differently. Personally I'd go to Oban, and stay somewhere nice like Lerags House (much recommended by various friends of mine, very aviation friendly I understand). You then have the option to assess the options (esp. weather, which will be like nothing you've seen!) locally before setting off for Mull, ideally in two trips. Getting in can be just as challenging as getting out!