Thermostatic shower mixers range in price from £15 (Lidl special) to £400 (Aqualisa Quartz). Obviously this price difference isn't down to just quality of materials and workmanship. I assume that the more expensive ones operate on a different basis than the cheap ones.
I would like to think that the more expensive ones would be able to cope better with other family members flushing the loo and turning on taps while someone is using the shower. Is this a fair assumption?