Thermostat valves for showers

I am looking to install a thermostatic valve for a gravity fed shower.

I am puzzled by the fact that nearly all the valves I have seen in the showrooms have only one control knob. I would have thought that you would have a control knob for the temperature and one for the water pressure you want.

Am I to understand that the the single valve is in fact for the temperature and you just dont bother controlling the pressure on these devices?

Ben

Really?? I have found most showers do have dual controls for temp and flow. I know a lot of obscure foreign stuff has invaded the showrooms though.