I've posted about this before and I haven't been to Mull, but if you had seen the Warrior nearly fail to get out of Eggesford 4-up a couple of months ago you would be thinking even harder about this. I know you're not foolhardy, but I see a hell of a lot of critical numbers in your flight planning. Good luck to you, but I like a lot of margin myself, not a little.
Why overstretch yourself on a first long trip? Plan to take one friend and plan to stop at Carlisle for fuel. With no offence to Warrior lovers, these things are dogs at gross weight and three passengers who don't know about aviation is three people to look after and calm down just when you don't want to, just when the weather is going down in Mull.
Don't make this into a marginal aircraft for this trip. Be kind to yourself.
QDM