The couple, who had left behind their son Eric, couldn't have looked happier at the bash, ahead of his big day on 7 October, arriving arm-in-arm and smiling.
Simon Cowell's actual birthday is on 7 October
All eyes were Cheryl Fernandez-Versini, formerly Cole, and her new husband Jean-Bernard. With JB's protective arm around her, the X Factor judge was beaming from ear-to-ear and looking sensational in a figure-hugging cocktail dress.
Also attending were Louis Walsh, Amanda Holden and Alesha Dixon, who posted a picture on instagram captioned: "Date night". The image showed Amanda's husband Chris Hughes and Alesha's partner Azuka Ononye.
David Walliams turned out with his beautiful wife Lara Stone, arriving with what looked like an expensive gift in hand.
Cheryl Fernandez-Versini attended with new husband Jean-Bernard
According to sources, the comedian made a typically funny and eccentric speech, joking that he and Simon are lovers, even going so far as to say that Eric is a cover for their love. Early on Tuesday morning David tweeted that he's unlikely to be invited back to judge Britain's Got Talent after the speech.
Simon's ex-girlfriend Sinitta paid him a more emotional tribute, saying in front of guests that he is her best friend, she is happy to be close to Lauren and that she loves Eric, her godson, very much.
The mogul famously stays friends with his former loves. Another of Eric's godmothers is Terri Seymour, whom he also dated for many years.
In a HELLO! exclusive, the TV presenter recently announced that she's expecting her first child with 24-year-old boyfriend Clark Mallon. Simon was one of the first to find out the news and was very excited.
"He’s so thrilled for us," said Terri. "We were having dinner in LA and I told him and he was over the moon. Actually immediately after congratulating me the first thing he said was he had to be the godfather.
"Then he said it was perfect timing with his son being so young and he hoped I would have a baby girl so he could set her up with Eric and they could date. 'How perfect would that be,' he said. Typical Simon."