Dress Code at Costa Coffee

No, they're getting strict on piercings. Only allowed small lobe stud.

I've been informed that a skirt is acceptable to wear as long as it is an inch above or bellow your knees. This is to avoid a burn hazard (upper legs from a short skirt) and a trip hazard (too long a skirt can apparently be a trip hazard...), as long as you still wear tights and socks to protect your ankles a skirt is acceptable.
This is under the new(?) guidelines from autumn 2018

I wore black jeans and top and a smart office jacket.

All black with costa top, apron, name and barista badges.

Get trained, finish your books, brand standard

It depends on the store / manager. My store allows most clothes (outside of ripped jeans or leggings) but others may not.

Depends on the manager but supposed to be none

You have to wear plain black shoes

Yes always

Yes aslong as it is black and your apperance is neat