Or any source of reasonably priced 800 x 800 square shower trays would
also be helpful just in case I can't get an 800 x 800 x 200?

I'm not sure why the height is important to you.

http://www.plumbworld.co.uk/55-1971

I have a Coram Waterguard shower tray. This is a plastic one on legs, so it
much lighter than the traditional stone type, making it a one person fit. It
is probably not a million miles from 200mm high. Mine is 900x800, but I
think they do loads of sizes.

The trap fits underneath the tray with no need for underfloor space. Very
easy to service. As it happens, I needed a large void underneath my tray for
a Sanishower and didn't want to bury it in the floor.

Christian.