Taking a cold shower can be invigorating and is said to be great for your hair. Taking a cold shower involuntarily, however, is an entirely unpleasant experience. If your shower starts getting colder and you’re not able to turn up the heat, it’s possible, even probable that the thermostatic cartridge needs replacing. These are sometimes called thermostatic cores or thermostatic cassettes. The vast majority of modern showers in the UK have these, and replacements are readily available. It is a little bit tricky to find the correct replacement as there are no standard cartridges linked to any particular models of shower, but if you take the old cartridge out it will be possible to identify the correct new one from a photo. The Shower Doctor website has a very useful section for this; you can find it here.
As always, if you would like a professional opinion on this, do give your local plumber (that’s us at Hedgehog Plumbing if you’re in Bristol!) a call and they help you out.