Flexible tap tails with valves

Well, a pressure reducing valve isn't just a flow restrictor. You can get thermostatic shows designed for the differential and varying pressures from a multipoint/combi,

but possibly not as part of a tap/shower mixer.

I`m not sure what the unit was exactly, a white plastic lump with what appeared to be a diaphragm and an adjusting screw underneath. I don`t even know if i`d need something like that in place to make it easier to control shower temperature (the cold has a much greater flow rate, so the ability to reduce flow might make it easier to control) - any loss of cold water flow to the bath itself wouldn`t be an issue as far as i`m concerned.