Well I love the look you have right now.  So personally would chose a natural stone or tile very similar to current wall colour.  As your wood units are such a gorgeous rich tone I thibk adding anything mid to dark would detract from these. On another note can I ask about your crittall style internal doors please? Are these actually metal? Can I enquire about supplier you used/cost as it is the exact look I am planning for our house. Thanks.

I have to confess it’s a nuisance when manufacturers produce worktops in one size and then upstands and splashbacks in another. If you are having the splashback all the way along the wall which will also go behind the hob you could have something different behind the hob which would give you the additional length you require. Usual hobs are 600 although some are 900mm. Perhaps have a contrast behind the hob? Premier Range have a webiste with some lovely options on. Bearing in mind you have chosen a neutral light grey you could successfully have anything with a hint of colour.

Going bold is one option, however i see a  very restfull, clean design. My thoughts immediately went towards continuing with that feel with a tile with only a soft hint of color, a tile with an uneven surface to create movement, or a tile with  'mother of pearl' finish on a soft white to be all color yet none.Your indoor plants will bring live color and not be overwhelmed on busy counter.

There are a huge array of tiles you could choose. Even metro tiles can look good if you choose a nice finish like Zelig, first pic below. Go onto Pinterest or Houzz photos to give you more ideas. I would take them up to that wall unit you have and behind the cooker go up to the hood.