In a bathroom the enemy is moisture, not light reflection — so pick a paint formulated to cope with humidity and condensation, and the sheen question sorts itself out.
For walls and ceilings, reach for a bathroom-specific finish rather than standard emulsion. Dulux Easycare Bathroom and Crown's Bathroom range both have anti-mould additives and a soft sheen that beads moisture rather than soaking it up. If you want the look of a flat matt but still need durability, Little Greene Intelligent Matt Emulsion is the one I'd point you at — it's a properly washable, moisture-tolerant matt that looks like a normal flat finish but takes the punishment. Earthborn Claypaint is a lovely breathable option for a well-ventilated bathroom, but I'd keep it away from the splash zone right behind a basin or shower.
On woodwork — skirting, architrave, window reveals — go for eggshell or satinwood. The slight sheen sheds water and wipes clean, which a dead-flat trim paint won't. Little Greene Intelligent Eggshell or Dulux's water-based satinwood are both solid here.
Now the question everyone asks: *can I just use bog-standard matt emulsion?* In a powder room with a window and decent airflow, you'll often get away with it. But in a family bathroom with a power shower and poor ventilation, standard matt will grow black mould in the corners within a year or two. Don't risk it — the bathroom formulations cost a few quid more and save you a repaint.
Colour-wise, soft and pale tends to flatter a steamy room. Farrow & Ball Pavilion Gray gives a calm, slightly cool greige; Little Greene French Grey - Pale is a gentle, airy choice; and Mizzle is a soft sage-grey that's a genuine spa-day favourite.
Practical bit: extraction beats any paint. Run the extractor fan during and 15 minutes after a shower, and even the best bathroom paint will last far longer. Sort the ventilation first, then the finish does the rest.