How do I bathroom / kitchen / extension? etc.

If you're after a non-ogee profile, probably one of these sorts of things. I'd say you would want something taller than 45mm, it'd look kinda odd. For ref, skirting is usually 120 or 144mm high.

Unless your plastering/tiling is already down to the floor, then you could leave it without and see how you go without any sort of baseboard at all, but you might find it gets kicked and cleaning around it would be a pain without marking the wall.