Why does my shower go cold when someone opens a tap?

A plain mixer holds a ratio of hot to cold, not a temperature. Open a cold tap elsewhere and the cold side of that ratio drops, so what arrives changes — usually hotter, sometimes suddenly.

What a thermostatic cartridge does differently

It senses the outlet temperature and moves a piston to rebalance the two supplies as they change. That is the entire reason the part costs more, and it is what a “shower that no longer changes when the toilet is flushed” describes.

What it does not do

It does not create pressure it has not got, and it does not work below the minimum pressure the maker states. On a supply that is too weak, a thermostatic mixer changes the problem rather than solving it.

The other cause worth checking

A combi boiler or instantaneous heater that cannot keep up with two simultaneous draws will drop the hot side instead of the cold. In that case the shower goes cold rather than hot, and no mixer of any kind fixes it — the limit is the heater.

What to do about it

If the change is hot, a thermostatic cartridge is the answer. If it is cold, look upstream first.

This does not replace the manufacturer's own instructions. Every dimension, thread size, flow rate, operating pressure, water seal depth and load figure on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A flow rate is reported with the pressure the manufacturer measured it at, and is never converted to another pressure, because no conversion exists. And the statement repeated rather than assumed: the 38 °C stop on a thermostatic mixer is a mechanical detent you can deliberately push past, not a scald-prevention device. What your installation needs is answered by the mixer's own instructions and by your national plumbing rules, and nothing here replaces them.