Hello Tom, I am no expert but I will try and help.
Low pressure gravity relies on water stored at height that then 'pours' through the household system. Think water tank in the loft, so not us. The original elson water heaters are of a open vented design fed with mains pressure water, according to the manual.
Thus the shower is mains fed via open vented electric heater.
Hope that helps.
All the best