We had a loft conversion done a few years ago. As part of this we had a thermostatic shower (bristan quadra) put in the loft ensuite and and a bar thermostatic shower put in on the first floor. These are both fed by a 2 bar ( I think) pump which is next to the hot water cylinder on the first floor. It took a while to get all the tiles done which probably did help matters. Plumbers subsequently got fired by the conversion company.
As such the loft shower never quite gets hot enough. It's fine for me if no one else has used the hot water but not hot enough for my wife. Conversely the shower downstairs is generally too hot. If you turn the temperature down the shower just shuts off its flow. You can then restart the flow by increasing the temperature knob and trying to tweak it slowly down so you don't get scalded.