We have a dark green slate floor in our downstairs loo, also London, and no limescale marks so far (been in about 3 years). It's lovely. I wouldn't use it all over though as it's very dark (though see my wet room comments below).
If you go for slate, I would absolutely definitely go for a honed (smooth) finish rather than those hand split ones you linked to. Anything with a rough surface will attract dirt and mould and gunk and be a complete b*gger to clean.
If you have no natural light, I'd probably go for light tiles but with markings in them so as to hide day to day dirt/scuffs. Beige is a bit depressing though. Pale grey or even white with marks is good. We have just put these tiles on our bathroom floor, in the whiter version - they are white with lots of grey markings and look a little bit like distressed painted floorboards (but only a little bit iyswim). I love them.
Or you could go very dark and dramatic, especially in the wet room, and use clever LED lighting to create a sort of underground spa effect.
Bathroom suite - as simple lines as possible. Definitely use wall hung loo to free up floor space and make for easy cleaning. The Villeroy and Boch Subway 2.0 compact loo is great for small spaces - very small but still comfy and a very neat shape.
You need some built in storage of some sort. I'd personally avoid those under sink vanity drawers that are everywhere at the moment as they are all veneer and once the veneeer chips will look awful. If you are building new stud walls, your builder can build tall thin shallow cabinets between the studs which are just deep enough for a shampoo bottle/make up etc. Similarly, build a niche/alcove into your shower wall for shampoo bottles.