I'm about to replace a poorly shower with a new one. I see in the
instructions that the shower head mustn't reach below 25mm above the bath.

With the way members of the current family shower, ie with the ability of
moving the shower head by hand this has led to some opposition where the
shower head should have limited movement.

I am aware that the reasons cited are to stop non-clean water from entering
the cold water system.

However if I fit a double check valve in the flow to the shower, won't this
remove the rules regarding the shower head and height of bath? Would a
potential reach into the bath then be allowed?