Sheen is simply how shiny a finish dries — how much light it bounces back. The trade scale runs roughly: dead flat → matt → eggshell → satin → semi-gloss → full gloss. Two things move with sheen, and they pull in opposite directions. The shinier the finish, the tougher and more wipeable it is, but also the more it shows up every lump, roller mark and wall imperfection. Flatter finishes hide sins beautifully but mark and scuff more easily.
Here's how I'd carve it up.
Walls and ceilings: matt, every time. It flatters wall imperfections and gives that soft, deep colour you're paying for. Farrow & Ball Estate Emulsion and Little Greene Intelligent Matt are the classics; Mylands Marble Matt Emulsion is gorgeous too. If it's a kitchen, bathroom or busy hallway, step up to a wipeable matt — Dulux Easycare, Crown Clean Extreme, or Little Greene's Intelligent range, which scrubs without going shiny.
Woodwork, skirting, doors: eggshell. It's the sweet spot — enough sheen to wipe clean and shrug off knocks, but soft enough to look modern rather than glossy. Little Greene Intelligent Eggshell and Benjamin Moore Advance are both cracking on trim. Avoid full gloss on skirting unless you're going for a deliberate period look; it dates fast and shows brush drag.
Bathrooms and kitchens specifically: the moisture-resistant ranges matter more than the sheen number. Earthborn Claypaint breathes brilliantly in older damp-prone rooms, while Dulux and Crown's bathroom-specific emulsions resist condensation.
The "but what about" question is usually radiators and metal — for those, Dulux Heritage or a proper metal finish over a Zinsser primer.
Practical advice: never mix sheens within a single wall plane, and always test your sheen as well as your colour. A matt and an eggshell of the *same* colour read as two different shades in raking light. Order both finishes in sample pots if you're unsure.