Hi Irina! That's a really lovely kitchen, I think perhaps using a natural material with variation will be an excellent finishing touch...have a look at our New York Brick tiles, we keep these in stock in West London can send a sample if you think these could work http://www.stoneandceramicwarehouse.co.uk/wall-tiles/floors/item/new-york-bricks

You don’t fave to have the same height all the way around the kitchen. You can have a 10cm upstand around most of it and then taller behind the hob. Personally I like the splashback behind the hob to be wide so make it the width of the shelves or the wall from the window to the corner and like Isla says make it the height of the shelf. Alternatively match its width to the wide cupboard and two small ones under the hob and then height size again shelf height or 30cm

Hi! Not keen on metro tiles to be honest although the narrower vertical ones aren't as bad 🙈 Just thought of a nice shaped one but struggling to know what colour works with a marble worktop 😂

I’d do differently. I’d tile up to the top of the units making a straight line, that way would feel nice to the eye, with a good flow feeling. To add to the look, the shelf will have a lovely background when you are setting your plants, herbs or spices… . I’d actually install tiles in a light colour with mate finish.