So, you are saying that there is or should be enough pressure to flow out of the shower (at least)? If you put your finger over the supply pipe can you feel "pressure".
It seems to me that the solenoid valve isnt opening and ditto problem with the last shower.
Did you check for 239V with the solenoid energised.
Maybe worth just running a supply to the shower via a extension lead or whatever, switch the shower to cold, switch off the the power supply to the shower at the consumer unit, MCB/RCBO or whatever, then connect in the temporary power supply to the shower terminals and see if you get (cold) water from the shower?.