I like option 2. Make a big statement--obviously you love carrara marble so go for it! You won't regret it.

Very interesting comments Bagpuss, thank you. I guess its not too late to change the furniture order! I would have never thought to use ivory furniture so that's definitely something to think about and I'm all for bringing in more light. I am planning ivory furniture for our living room, hallway and bedroom too (I love ivory furniture), so thought we'd do something different in the dining room. The walls are light too (off white/ nearly ivory) - that's another issue as I'm not sure that colour is suitable or needs to be changed. I actually thought (hoped!) the Mango furniture would create a contrast with the flooring as opposed to a clash and there are some (albeit few) darker tones in the mango furniture that match the flooring so there is a bit of a connection (or so I thought). And it sounds like I'll definitely need to rethink the fireplace altogether!

There are a few options but it's a tricky shape to get too cheaply. I have added a few 'massively' cheaper alternatives too. http://www.bathroomcity.co.uk/product/lola-1700-x-750-freestanding-bath-inc-chrome-overflow-and-waste https://www.bathstore.com/products/london-free-standing-bath-576.html https://www.bathstore.com/products/wash-freestanding-bath-with-surround-panel-1700-x-750-1739.html  I hope it helps a little.

Rather than use a local joiner, you could try bathpanelstore.co.uk, which makes custom panels for very reasonable prices (and where I turned when I needed a new one.) A plain single piece MDf one is about £70, and marine ply ones are £106. Making them two piece / with a plinth adds about £15 to the price. You can send them a drawing of what you want and they will make it. So given you can’t accommodate drop down hinged doors, how about pop/ lift out doors? They can be secured using roller catches or magnets. Depending upon size and placement, you could probably figure out a way to maximise access to the side of your bath that has the tower cupboard next to it. If if were mine, and I was feeling a bit handy and creative, I’d order an unfinished marine ply with two concentric rectangular cut outs for each desired door... the larger cut out would be the lift out frame, and the smaller one would the centre door panel. Then I would get all pieces shipped to me (they will all fit together like a puzzle.) I would then stain them to coordinate with my bathroom, making sure to read up on how to prep ply for staining. I’d then fit the fixed frame panel, shave a small tolerance off the lift out frame, and fit magnets or roller catches so it can be easily removed for access. And finally, fill the centre opening with either the original panel (whole or cut into slats), or perhaps wired bamboo/willow/reeds edging from a garden supply store. Poundland has them right now, too. Or get the thin fretwork panels used as radiator inserts. Or heck, even a bunch of thin shell necklaces like a Moana-inspired fringed curtain. Thanks for letting me live vicariously