I have young kids, so this issue recently came about for me too. I have been using simple digital thermometers from the pharmacy, but question their accuracy and find the battery life is poor. It seems shortly after replacing the battery, they begin to malfunction. I searched for a quality thermometer, but the ones like the doctor's offices have are very expensive, and I notice the nurses often fiddling with them and having to exchange for another when they aren't functioning. I don't know about the UK, but I don't think you can buy mercury thermometers anymore. I wish I could, as I'm sure my mother still uses the same one from my childhood. I have settled with buying Vicks brand (like the vapor rub) digital thermometers, keeping a couple on hand in case one quits, and treating them as semi-disposible. They aren't very expensive here, and those are the ones we've had the best luck with.