Triple S in Bingley do excellent powder coat in metallic and candy finishes. You are right, they do a base PC then top coats to provide the metallic sheen or candy finish. It isn't cheap but it looks awesome. Their standard finish is quite shiny for PC anyway.
It's a pretty standard way of doing it - car paint is the same - regular paint then metallic lacquer. That's why when you wash your car with washing up liquid or use T-Cut on it too much the paint goes full. No more lacquer!