Only 316 grade stainless steel should be used outdoors in the UK.
This does not just apply to signs. Anything decorative such as handrails or casings, or industrial labels with information that needs to be readable, should be made from 316 grade stainless steel.
The British Isles are a group of islands sticking out into the Atlantic Ocean, with a wet climate that no matter where you are, arrives from across the sea. Inferior grades of stainless steel such as 304 discolour and rust in this environment.
This makes signs look unsightly, and labels or nameplates potentially unreadable. We only use 316 grade stainless steel because there is no good reason not to find out more here.