The colors need to work with the furniture you plan to have there. If you don’t like gray - you probably won’t like ultra modern/ikea/metal furniture.

Other than that it’s a matter of personal preference for the palette. Given that you’re in the UK (enough gray sky) and the room is kinda darkish - I’d consider lighter shade of earth for the “darker” wall(s) and close to white, maybe with a drop of blue or green, for the lighter ones.
Before you go and buy lots of paint - get a few samplers and try them on each wall, try in a corner next to each other to see whether they work for you. Ask at least one other person for opinion when you do that.

Hope it helps! The room looks promising. You’ll enjoy it if you put some love in. :)