Bathroom combined plumbing, decor advice please

[ClutterofStarlings]

Long story short, new house, short of cash. Can’t afford to get bathroom done, so diying it.

There are three types of wall - 2 different tile styles (one pale browny grey, one pinky)plus wet wall in a totally different colour round the shower. The floor has quite nice looking but freezing cold tiles.
The whole bathroom is cold despite being south facing and a small room. Two external walls and a large window.
Room dimensions 174x 186 cm

Since the shower leaks, and the small children need a bath, I am going to put in a bath.
I would ideally like to fit some insulation over at least one external wall, the other is mostly window & has the sink & loo in.
Must I tile over the top of insulating board, or can I waterproof and paint somehow? This will be behind the shower, so needs to be pretty watertight. I’m a bit concerned that insulating board with tiles over the top will lose us too much space.
What is the cheapest way of tiling? I’ve found some lovely (big) tiles on gumtree, but I can’t find any to match & they would only just probably do the area by the bath with no spare for errors.
I’m considering some cheap cork tiles for the small floor area & maybe getting a small underfloor heating mat. Then I also get to considering the ceiling & wondering if I should take that down to ensure it’s properly insulated….also to get rid of the manky ceiling tiles & light.

Plumbing questions -
I think I can do the bath, but is it a case of taking out the current fittings and the deciding what to buy (eg How will I know if I want a shallow bath trap or a low level bath trap? ) or can I tell before I take out the shower tray and wet wall?

It all seems to be so interlinked I can’t work out what to do first/at all/what yo not bother with. I don’t know whether to make it a huge project (difficult because of time constraints) or just do the bath which will be a moderately sized project of its own. But then, if I want to insulate that wall, it has to be done before the bath goes in….and it would be nice if it looked nice.
The only thing I’m set on is the bath and a storage bath panel.

If anyone felt like offering any thoughts I’d be most grateful.

I think you need tile around the bath - you can get a type of plywood called MarinePly and then tile on top of that.

Big tiles are harder to do because they are heavy & can fall off the wall easily. Also, fat tiles are fashionable but are also heavy. If you can, get some thin classic tiles that aren't too big.

Okay that’s interesting, I thought bigbtiles would be easier as there would be fewer joins! I had heard of marine ply, altho hadn’t really looked what it was & was going to do the bath panel out of that. I will need to see if it has any insulating quality.

Of course the other problem as always is I want this done yesterday, but I’m trying not to rush.

Anyone else have any thoughts?

I’ve been looking at screwfix forums who suggest insulated ply and “d&d” this straight to the wall, taking off old plaster, but I don’t know what d&d means and am struggling to google without getting dungeons and dragons!

I was vaguely hoping @pigletjohn might be about for the plumbing questions. I’m assuming that if I get behind the wall and discover that the pipe types are different I could get converters?
Having said that if it really is a disaster we can probably afford to get someone to sort it out. It’s all going to be fine.

I guess also how much difference will insulating just one wall make, plus or minus the ceiling as the other external wall is mostly window and has the rest of the suite on it.
I’m sort of thinking some insulation is better than none?

Dot and dab. It means that you wouldn't spread adhesive all over the surface but put it in, say, the corners and a couple of middle spots of the piece you're fixing up.

Thanks, I’ve finally managed to find it! But sounds as if it’s somehow harder than battening which seems unlikely? But also sounds less secure than battening for sticking tiles to.

Modern practice is to line bathroom walls with cement-based waterproof backerboard before tiling. You can get a grade with a rigid insulating backing. A good modern tiler will be familiar with it. Hardie is a good brand.

You can strip off any old cladding or defective plaster first.

Plasterboard and ply do not withstand water.

That’s great thank you. I think DH wants to try at the tiling himself (I have my doubts! ) I couldn’t find a product that seemed to be insulating with a good enough surface to put tiles on.
I’d been finding out about micro cement which looks lovely but seems to cost a fortune, so that’s on the back burner!