Try a wall mounted sink basin and a toilet that doesn't have an elongated bowl. These will save space.

i have a shower bath....my oh loves a bath, me and the kids prefer showers so we"re all catered for. just had a total bathroom refit and I love it so much, its very modern looking, glossy black vanity and bath panel ,glass and chrome tiles, black flooring and loads of mirrors.....

I think the smallest size shower is 700 so would probably use that as a minimum although that's for a fully enclosed shower so you could potentially go a tiny bit smaller if stuck for space. It does also depend on your family's size though as to what is comfortable for everyone. I'd also include the niches for shampoo bottles etc if possible

Because the Mira round thermostat/ controller but is set into the tiling would it be easier and cheaper to look up the shower you have and replace like with like? It this it? https://www.plumbworld.co.uk/mira-excel-thermostatic-shower-19352-15218?msclkid=f4ee760811121c2d74982861331525c6&utm_source=bing&utm_medium=cpc&utm_campaign=Shopping | Supplier | Mira&utm_term=4578229006415360&utm_content=Ad group #1