The honest answer is that it depends, which is why a quote is worth getting rather than going on a headline figure. Price is driven mainly by four things: your age and driving experience, the vehicle you're covering, where you live, and, for other durations, how long you need cover for.
Looking at Covertime's one-day policies sold between January and June 2026, 25% of customers paid less than £20.96, and half paid less than £26.81. There's a genuine spread either side of that too. Some customers paid closer to £17, others closer to £51, depending entirely on their own circumstances. That range is normal and expected - it's exactly why two people covering the same car on the same day can end up with different prices.
None of this tells you what you specifically will pay. Your age, your car, your postcode and the day you need cover for will all move the number, sometimes by a fair amount. The only way to find out where you land is to run your own details through a quote.