The short answer
Research on UK automated valuation models puts them within 10% of the eventual sale price roughly 70 to 80% of the time, so about one in five estimates is more than 10% out. No consumer portal publishes the realised error of its own estimates.
Accuracy falls apart where public data runs out: condition, layout, aspect and a recent refurbishment appear in no register until the property sells. That is why a single number is the wrong output. A range, with what could not be read named and the gaps priced against it, tells you how much to trust the figure before you act on it.
Why this matters for your purchase
This is the kind of thing a PropM8 Scan is built to show you early, before solicitors, surveyors or lenders are involved, while walking away still costs nothing.