Homebase does some great porcelain tiles - Luna. The beige (also available in black but won't recommend) one will work well with your units I think. You can use the 300x600 size for the wall and the 400x400 for floor. They are not expensive at all, we paid only around £12/m when we did our bathroom in the old flat, hope the photos below show. Be aware they are very difficult to drill though. The word porcelain says it all! :). For the splash black though, it's been my dream to have a coloured glass one, any colour will go well with your neutrally coloured units. So if you managed to save a bit on floor tiles, I'd use the savings on glass. All the best!

Fishscale tiles are very Moroccan, especially in a turquoise shade. Mexican in much more colourful and patterned. Have you looked on Pinterest? Great place for ideas, as it Instagram. Here’s some ideas

If you want to modernise the kitchen then yes, knock them off, reskim the walls and have no tiles but that’s where you start to think where does the spending end because it looks nice with upstands which then means swapping the worktops so they’re both the same colour. Those fascias are from an old design range. You could keep the existing carcases as long as you like the design of the original kitchen so just having new doors is the cheapest way.

The floor tile is fine, though I'd choose the right worktops for the kitchen (with upstands) and choose the tiles afterwards if you're not sure. Personally I'd go for a medium to heavy veined white quartz worktop, then probably a plainer tile. Or even better continue the worktops up for the splashback ... though this maybe alittle higher on cost. Also a stain is a stain no matter what colour your worktops are. They need to be looked after like anything, but are generally very good. Good luck with you choices