The composition in this shot needs some work. Personally I think it should be either the telephone box or the letterbox, not both. Secondly, you shouldn't have main compositional elements in the centre, they should be off to the sides to satisfy the rule of thirds. Since the surroundings of the telephone/letter box are quite dull, I would have less of them in the shot. Finally, the sky looks quite blown so you might want to try and recover some of it in post.
Its good that you're exploring selective colour though, lots of good photo opportunities there. I'd just steer clear of phone boxes for the time being cos they have literally been done to death