Small bathroom - feasible layout?
Is it an option to have pipes above floor level but boxed in


Yes.

But if the hot and cold water supply pipes in and are metal (copper) and have supplementary equipotential bonding - I think you need to make sure the bonding clamps are easily accessible without tools. Ditto for isolating valves (for when you want to replace taps).

You don't need much slope on the horizontal runs of the waste pipe (and too much slope is bad) I read 1 in 40 to 1 in 110 somewhere.

I've boxed in pipework minimally but if I did it again I'd try to make more of a feature of it - cupboards with useful storage space as well as access to pipes.