getting fuel when you can rather than when you are nearly empty has further importance in the Highlands. If fuel prices in urban England feel high, in rural Scotland they will make you cry! Prices vary massively and you need to give it some thought.
Rule number 1 - never, ever plan to fill up at a motorway rest break area. They are there for people who are charging it to somebody else, or those who really shouldn't be driving!! You won't believe the prices until you see the signs....
Rule number 2 - in general the big supermarket chains at stations attached to their large stores are often cheapest, especially if there are three or more in a close area. But if there is only one or two, or if it is a smaller convenience store this may not be true. And cheapest is a relative term...
I did a price comparison for you plan but the system won't let me post that many numbers. But entering the Highlands with a full tank makes sense in more than one way. And leaving York with a full tank makes sense today. Prices in your travel areas vary by over £0.20 per litre that's a difference (not actual price) of $1 per US gallon- and more if you use a motorway site.
Use this to locate fuel and check prices, https://www.petrolprices.com/app/map check the "updated" date on each station as prices change daily at the moment (and guess who is to blame for that.....!) And don't forget, green pump handles are unleaded (E5 or E10), black are diesel. And the reference price to keep in mind is £1.579 per litre - because today that is almost exactly $8 per US Gallon.