yeah, cant you put the iso valve in the loft or under the bath,  depending on how you are running the pipework. I imagine you are teeing into the main cold water supply somewhere, putting the valve in there would do, if its accessible(you know where it is)


btk

get it inspected pre-tiling, tell the inspector its getting an access panel.


btk

if the shower head cannot be immersed in water you dont need a check valve,  however, if you later install a longer hose it might be, and if you are running from the mains the check valve wont restrict the flow too much, in fact i fitted a mixer shower today which came with check valves on both hot and cold(gravity) and the flow restriction was negligible.


btk