Shower over bath - is a mixer required?

Mira 415 combiforce is I believe pressure balanced although not thermostatic. If you just have a normal bath mixer diverter, you will find that if someone turns on a cold tap, instead of the pressure dropping and the temperature remaining roughly the same, the temp will suddenly get very hot. Could be dangerous, although some have a max temp shutoff. If hot tap turned on, shower may go very cold. It's possible to have a separate pressure balancing valve on inlet to the bath mixer in you need it. I have a thermostatic bar mixer fed via a pressure balancing valve. Best of both worlds. Of course you can get very expensive showers that do both.

Plain mixer, if its just you in the house, may be OK, although don't use the washing machine when in the shower ! Simon.

We used both and found the controlled one was a pleasanter experience when the inevitable knob ran a tap. (The screams of rage from the bathroom disturbed the peace and quiet)

If the boiler is working properly and the supply uninterrupted from elsewhere the ordinary mixer is fine.