Not quite sure what you want here but there are thermostatic units that fit to either shower direct or to mixer taps on a bath Lidi sell them from time to time are available. Nothing to do with electrics though so not sure why you have posted here? Electric showers are fully self contained and only have a cold water supply. Normally 7 to 10Kw where as those connected to combi boiler are more like 27 Kw and as you might expect are a lot better. With a pump off a storage tank more like 40Kw with side spray units the bees knees but as you may expect they cost more than electric to fit. All plumed in units need to take water from same pressure so with combi easy as all at mains pressure but with pumped off storage tank the cold water needs to come from storage tank not from mains. Even with electric there is a problem in routing the supply cable which is rather thick they need their own dedicated supply and can't be connected to ring main etc. With this in mind can you say what your looking for?