For concealed pipework use the 3/4" male iron (which are brass) to 22mm compression unions supplied, put the unit the way up which puts hot on left cold on right. Put enough ptfe on the elbow thread (10 to 15 turns, most towards back). Short stub of pipe into pushfit elbows pointed to where the pipes are, I think you are saying they are in the loft.
It's tricky to do up the compression nut as it sits back in the wall, so you have tyo dig out a large enough chunk of wall to allow you to pull the pipe forward for getting onto the comression nut, yet push it back sufficient afterwards for shower to be flush with wall. It helps if during first fix you use compression stop ends so that the compression nut has already bitten , then when you come to fit shower after tiling, if you lack confidence you can paste the olives and tighten up, but don't have to go at it as hard as you do when making olive bight on a new joint. Check for leaks before fixing shower back.
If you need to ask these questions you are about to land yourself in a deal of trouble. I had a lad working for me with three years experience and all his qualifications and he phoned me to say it couldn't be done. This after the tiler had been and gone. Anyhow as the boss I have to make sure it is done, it was done.
One of the hardest showers to fir concealed if you have no access from adjacent room/ cupboard. A lot easier to send chrome pipes down from loft and fit exposed pipework.
