I love those long floorboards/wood-look tiles and they are the feature so you wouldn't want something too busy on the walls... personally I would go with a larger format say 300x600 rectified edge white tile with a matt finish for the Hamptons look... what's your vanity?... a timber one would tie in nicely with the floors and all the white... very Hamptons :)

depending on the size of the existing vanity it should be possible to have a similar new vanity or a custom unit made to replace the original but a chat with a plumber should be arranged to recommend suitable options and plan a schedule to remove the old and install the new, including any necessary waterproofing or tile repairs..btw careful removal could allow the original sink, taps and even the counter to be saved and recycled into a new vanity that matches the bathroom perfectly with the added option of being able to add different doors or drawers to the cabinet

How big are the floor tiles? It may look unbalanced with a more delicate tile in the floor and the 600x300 on the wall. Are you able to continue the flooring tile up at least one wall for cohesiveness.

I would prefer to keep the WC separate to the Bathroom as you have 3-4 kids? who will get bigger very quickly, having a powder room downstairs won't help with the morning rush to school., But it's your house so I have added a pocket door between the WC and Bathroom. I have also taken that small robe space for an extra vanity in the WC/Powder room. No width dimesion given for the WC The ensuite could use that extra entrance space as a sit down makeup and hair bench for your wife.