There is a zinc roofing association. Ask them.

Hi, Extension plans are moving on and now trying to finalise which roof finish is best. We have looked at various different options but wondered if anyone on here had any personal experience or tips. The roof will be slightly slopped. Main house is grey tiled and white render. Extension will be cladding (dark grey or black).

We have render and some shaped timber

i prefer stainless steel but only if undermounted. Specialist steel cleaner or bar keepers friend is great for keeping them spotless. You have to accept the scratches though or be very careful. Limescale is no problem and we have exceptionally hard water here in the South West. Limescale is more of a problem with taps even the Quooker has to be serviced once a year and limescale removal is part of the service.Blanco make exceptional taps. My Culina S never had a limescale problem and you can replace the screw off end if necessary. It's also a dual function tap with sprayer.