Walls and Floors have these https://www.wallsandfloors.co.uk/white-tiles-white-floor-tiles-15072 https://www.wallsandfloors.co.uk/matt-black-white-floor-tiles-black-floor-tiles-15071

My bathroom looks to be about forty years old, judging by the dated tiles. The tiles are awful, but the shower grout looks fine! If you like the small format tiles, I wouldn’t necessarily assume they can’t also go in the shower.

Maybe some kind of tile trim? Most of them fit half under the tiles to secure them. Or plastic strips like window fitters use to tidy up gaps. Something like this but tile shops will have a selection.

Thanks for the advice everyone. The walls are just plastered at the moment, so I'm thinking to paint them a shade of white first to get a better idea of how the space looks, then decide on some tiles. It's difficult to picture with how it is at the moment! I do like the idea of having white zellige tiles placed vertically, but worried it might look a bit busy if it goes from the countertop to the ceiling.