I'm pretty sure our bathroom walls will be Prismatic oblong tiles in a brick layers pattern similar to this image on the left but I hadn't thought of adding a dark border to the bottom where the tiles meet. It looks more purposeful and thought out so it's certainly something to consider. I love the hexagon tiles but they don't seem to exist in this country so I am currently searching for an alternative.
Have ordered the sink, toilet, taps, shower rail and shower valve now I just have to sort out the shower head, shower curtain, radiator and tiles...