Hi

When I moved home, I had the same problem with a "twin control thermostatic shower valve" (unbelievably, the user's manual does not have the name of the manufacturer!). The minimum temperature was already too hot.

The thermostatic cartridge (that actually mixes the waters) is able to do 2 complete turns between hot and cold, however the handle can only turn once due to its safety lock (where you have to press a button to override the maximum temperature).

To fix this I removed the handle, manually turned the thermostatic cartridge to a "colder" position, and put the handle back.

This was a simple thing, without the need to close service valves.