All would go fine with your combination. I'd advise caution with the darker finishes if you are in a hard water area.

Hi, It's hard to tell without seeing photos of the tile & vanity unit colours. If you're going for a tonal bathroom look, a vanity unit in a colour close to the wall tiles will stand out the least and create a soothing look. I think you are right in thinking a dark grey unit will be too strong given the presence of anthracite on the window & towel radiator. Would you consider a wood effect vanity unit? They pair beautifully with grey and give the look warmth.

It depends on cost, how long you are going to be in your house, cleaning, and preferences. White is always the least contentious and easy to sell on in future. I do like the wood effect though, I cannot see many people disapproving of that. Its a really good contrast to your floor tiles. If you have hard water and no water softener I would rethink that black tap.

Are you removing the current basin? You have spent all the money on a bottle trap to be exposed and if it was going in a vanity you could have just had standard white pvc fittings.