The head judge unbuttoned his crisp white shirt for the glitzy bash, which was also attended by Amanda Holden, Alesha Dixon and David Walliams, and showed off his fuzzy body hair.
Despite the BGT final being interrupted by musician Natalie Holt throwing eggs at him, Simon seemed in high spirits and had clearly put the incident behind him.
Simon, his fellow judges, former girlfriend Sinitta and Ant and Dec partied the night away at the exclusive 45 Park Lane before heading home in waiting cars.
The BGT and X Factor mogul was then pictured laughing and joking alongside a mystery blonde woman, who was travelling in the back of his vehicle.
Following Saturday night's final (8 June), violinist Natalie apologised for throwing eggs, but described Simon as "dreadful".
He, however, has laughed it off, tweeting: "I don't think eggs should be allowed on talent shows. Discuss?"
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