Plumbing Questions? Ask a Plumber for Answers ASAP
What height from the tank is the pump currently fitted, and what is the current differential between the pump and the shower head (when are normal height).
If the shower is set to cold, do you still have the same issue with the pump kicking in unless you reduce the height of the shower head?
My guess is the issue is the Mira shower is restricting flow and therefore pressure which is preventing the pump detecting sufficient pressure drop to start, so dropping the pump wouldnt likely help as the pressure differential between shower head and put tank is likely causing your issue.
Would it be easy to raise the pump so its 600mm above the shower head, as I think if you increase the drop that would probably work?
Do you need any further assistance?