The Scotland trip - get things going

I have asked for holidays and put together an Excelsheet with all the dates and the locations. 

The whole trip will be done mostly by train to see as much as possible of the countryside and to enjoy a more old-fashioned way of travelling. After loads of to and fro, I have decided to travel clockwise with the following stops:
Brighton - London - Glasgow - Fort William - Dornie - Inverness - Edinburgh - York - London - Brighton (London and York will only serve as a stops to change trains.)
I am not 100% happy with this itinerary because the stay in Edinburgh will be shorter than I originally planned, since I only have 8 days at my disposal.
I can’t book anything yet, because I will be trouble if the holidays don’t get approved. The main highlight of the journey is looking good because there are still first class seats available for the trip from Fort William to Mallaig on the Jacobite Steam train. These tickets go very fast, so booking them is my priority.

To make matters more difficult, I have added further requirements to make this trip utterly individual.
I want to travel with Grand Central from York to London, after seeing one of their trains in King’s Cross station. Grand Central is a small train operator with limited services, but for some reason, they stirred my curiosity. And since I prefer to do things properly, I have decided that this trip is going to be done in 1^st class as well.