Cowell is party to stories associating him with women, so much so that I think he is gay. He can't sue here for libel if papers simply say he is gay, but he is such an important figure, so well- connected, so powerful in entertainment, and respected in the media' that it's easy to see why no national paper would wish to say so, and would respect his wishes.
Nobody says that Cliff Richard is gay, either.(Of course, there's always the possibility that he isn't!) His case is rather different, because when he started out homosexual acts were illegal and saying that someone was homosexual was commonly regarded as a slur. That's how Liberace was able to successfully sue for libel for the innuendo ("effeminate" etc) that he was gay.
Walliams is an odd case. We don't know the sexual preferences of his wife; there have been plenty of gay men who married, John Maynard Keynes, Jeremy Thorpe, Oscar Wilde and Elton John being examples from various times in the past. He may well be gay.