Maybe you're over-thinking it all a little. Yes, it's a good idea to have a rough sense of what kind of average speed you need to maintain in order to reach the controls before they close and have sufficient time in hand for eating, resting, etc., but (IME) trying to plan a 1500km ride to the nearest hour or two is fruitless. Terrain, weather, tiredness, lethargy, speed of service at controls.... all will conspire to upset your plan.
By all means, have some rough targets for reaching, say, Louth, Barnard, Brampton, Edinburgh in each direction, but if you fixate on them you'll not enjoy the ride so much, and what's the point of not enjoying it?