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.

It’s common for tiles to be slightly bowed. For this reason this brickbond layout of tiles is very unforgiving for these large tiles because you have lined up the edges that are bowed in with the middle that is most bowed out. Had they been small tiles you wouldn’t have noticed but the effect is exaggerated with large tiles. That’s why the manufacturer recommended an offset brickbond laying pattern. 90/10 would have looked great. Given your efforts to convey this instruction to the tiler it’s disappointing that a professional who should know it’s not recommended to lay large tiles 50/50 brick bond to avoid exactly this situation went and did it anyway. I think the tiler is liable because even absent your instruction he should have known better. It’s professionally embarrassing that he has done it. I think the contractor also shared some responsibility if they hired the tiler. Basically it’s their recommendation, their reputation that is being damaged by poor workmanship and their relationship with you that is at risk. Consider the paper trail - did you get a quote directly from the tiler or through your contractor? It may be that you get nowhere but you should try to get the tiler to accept responsibility. After an firm conversation you need to write to him to outline what you feel is his fault and clearly stating what you expect him to do in return (such as replace all the tiles). You also need to state a timescale for response from him. If you can get it changed on his dime then happy days. If you have to pay for it you might conclude that you can’t live with it. Personally I wouldn’t spend the money a second time myself and I would live with it.