Only local ones we use for work sorry.
The most Midlands-ish one we've used is
used are these guys. I'd rather not link our actual supplier as the address would pretty much help you find where I work, etc.

I think our two definitions of sheet metal may be different anyway.
I'm thinking folded parts - I'm guessing you need an actual supplier of flat metal sheets? I'll check with our fabricator tomorrow if you want? What material (Aluminium I presume? Steel is cheaper, although it is a fair bit heavier) with a rather thin gauge? (Thickness)