Metal railings live outdoors taking everything the British weather throws at them, so finish matters less than the system underneath it. Get the prep and primer right and almost any decent exterior finish will hold up.
Start by killing any rust. Wire-brush back to sound metal, then spot-prime bare patches with Zinsser or, for serious rust, use a rust-converting primer. For most cast or wrought iron, Hammerite direct-to-rust does both jobs in one and is hard to beat for railings and gates — it's a proper tool for the job, not a topcoat brand competing with your decorative paints.
For the finish itself, gloss is the traditional pick and still the most hard-wearing for railings. It sheds water, wipes clean, and suits period properties — most people picture railings in Off-Black or a deep blue-black, and Farrow & Ball's Off-Black or Railings look the part. If you want something with a touch more depth than flat black, Down Pipe (a soft graphite grey) is a cracking choice and reads less stark than true black.
If you prefer a quieter, more modern look, go for a satin or eggshell exterior metal finish. It still beads water but loses the high shine — flattering on contemporary balustrades. Inchyra Blue or Hague Blue in an exterior eggshell looks superb on railings without shouting.
The "but what about" question: can you use ordinary exterior eggshell? Only if it's rated for metal. Standard masonry or wood eggshell won't key to bare steel and will peel — that's why the primer is non-negotiable.
Practical advice: pick a dry, mild day, two thin coats over one thin coat any time. Mask the brickwork or stone with FrogTape, and get into the awkward backs and undersides — that's where rust always starts. Do that and your railings will go a decade before they need more than a wash.