I think from what you say that you will have an a light grey wall color from the high railing down, darker grey above the rail, and white on the ceiling? You would paint the fireplace surround the light grey color (Cornforth white) but paint the remainder of that wall darker grey (so below the rail as well), with white built-in cupboards on each side...is this correct? If so, it seems there would be a "stripe" of the darker grey around the room on the upper wall, not sure how this would look (a photoshop mock up would help, but I don't have that skill). I like the Cornworth color and think it looks great with white. I would use only the two colors - white for the trim/rail, the fireplace surround, and the ceiling, and light grey for the rest of the walls.

Thank you all for your comments. kl55hp your project sounds equally exciting. One thing that I learned so far is the need for light, big windows, big doors!! We have exactly the same dilemma regarding living in the house during the building work; it will save a lot of money as well. You have definitely thought this further than us. Sounds like there is a consensus regarding using a professional to help us make some decisions! OnePlan, thank you for the pictures the colours are spot on. Were thinking of a slightly more modern look by incorporating some gloss cabinets to reflect light etc. Thank you all once more!

I’m no expert but if you want a nice clean finish it’s best to get a plasterer to skim the walls. You could try some filler like Polyfiller in the dips, let it dry hard then gently sand smooth.

Thank you @User, I knew what you meant with endurance as well 🙈 Ive just read that with darker colours it changes the balance of the resin in the paint making it harder to keep clean and scuff free. Every day is a school day 🤣