You have a beautiful British kitchen vibe going on here. Don't ruin it with a flashy wall of tile. (Even a subtle tile is going to detract from your incredible marble stove wall splash.) . Leave things as they are and get a nice piece of artwork to hang to the right of the window.

Maybe too late but I would have used the counter material as backsplash on that wall, cut to be level with the sill

Same counter material.

I have tile on my window wall and the other 2 walls, but I did the same counter material under the window because it would have been a pain to cut/lay the marble mosaic in that small space.

We might recommend using the countertop material, if possible, in that area for a backsplash as well. If you cannot do that, you can look at different tiles online that might work for you. Here are a few of the kitchen designs we have done for clients, which could help you more with your current dilemma.

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