In a 2018 video interview with Vanity Fair, in which he discussed his most famous roles, Hugh Grant blatantly said that "Notting Hill" is based on a true story. "[Richard Curtis] wrote this other film and it was based on something that actually happened to a friend of his, who was a very kind of unfamous bloke living in Notting Hill, who happened to fall in love with an extremely, extremely world-famous person who I'm not allowed to mention, and she with him," he explained.
Grant mentioned Curtis' mysterious friend again in an interview with E! Online, revealing, "This is a story [Curtis] won't admit to, but he's told me in a drunken moment. A friend of his, an ordinary, normal guy, was in Harrods one day and met a very famous woman, and ended up taking her back to his flat in Notting Hill—and all kinds of nonsense ensued. And they used to meet up, whenever she came to London their affair would reactivate itself—and that was the genesis of his script. But he's so scared of people finding out who this very famous person was that he won't tell anyone that story."
Curtis himself has never spoken publicly about his friend, nor identified the celebrity in question. However, the Daily Mail ran a story in March 2021 alleging that the friend is a man named William Sieghart and the famous person was actress Uma Thurman. The Daily Mail didn't cite any sources and merely wrote "evidence now suggests" — so this element should definitely be taken with a grain of salt.
Source : https://www.looper.com/631286/is-notting-hill-based-on-a-true-story/292