I think being a very small bathroom we've decided to do all white matt wall colour and dark tiles for the floor. Let see how it turns out. We're going to add features such as lights and mirror with frames around it. I'll send before after photo.

There is no specific regulation but it would not be recommended behind a gas cook top. If there is a concern you could install the timber behind a glass splash back for protection. There is also a really nice tile that looks exactly like timber as another option.

your two colours don't seem to go..you need a different grey and a darker grey.. I'd go and grab some sample tiles and go home and pop them on the floor and see which ones go best with your walls.. tile shops are great they lend them out for a small deposit.. I'd take a photo of the tiles against the wall..sometimes our eyes lie..the camera don't ..once you've taken all the photos have a scroll through you'll be surprise which one will suit

Could you use the same glass as behind your cooktop for behind the sink? Kitchens generally use the same material for all the splashbacks or it can look disjointed. Have a look at photos of kitchens on this site to get some ideas.