Eggshell or satinwood is what I'd put on a radiator nine times out of ten. The old habit of slapping gloss on rads comes from a time when that was all that stood up to the heat — but modern trim paints handle the expansion and contraction of a hot radiator perfectly well, and they look far more current. Gloss shows every dent, drip and bracket, and it yellows over time. A flatter sheen hides the lumps and blends the rad into the wall.
The key thing isn't a dedicated "radiator paint" — it's using a proper wood-and-metal finish over a sound primer. Little Greene Intelligent Eggshell is a cracking choice: it's water-based, low-odour and rated for radiators and metalwork. Dulux Heritage Eggshell and Farrow & Ball Modern Eggshell also stand up to the heat once cured. If you genuinely want a gloss, go for Crown trim gloss or a specialist radiator enamel — but I'd talk you out of it.
Prep is where rads go wrong. Knock the heating off and let it go stone cold first — painting a warm radiator will leave you with a streaky, ropey finish. Degrease it (sugar soap), sand any flaky old gloss, then spot-prime bare metal with Zinsser Cover Stain or a metal primer to stop rust bleeding through. Skipping primer on bare or rusty metal is the number one reason radiator paint peels.
Colour-wise, most people want the rad to disappear, so match it to the wall. If your wall is Farrow & Ball Slipper Satin or Strong White, take the same colour into eggshell on the rad. Want it as a feature? A deep Inchyra Blue or Down Pipe radiator against a pale wall looks brilliant in a hallway.
Thin coats, two of them, brushing in the direction of the fins. Leave it a good 24–48 hours before you fire the heating back up — switch on too soon and you'll bake in a smell and risk crazing.