Sparking an all-out fashion frenzy, stylish stars turned out in London last night to help designer Matthew Williamson celebrate his collaboration with H&M. The British designer, who kept pal Sienna Miller close by his side, told PEOPLE just how nervous he had been while heading to the launch of his capsule collection with the cheap-chic retailer: “In the car on the way I said to Sienna, do the heart test, feel my heart, and she did. It was pounding!” But it turns out Williamson had nothing to worry about. As VIPs like Mischa Barton and Pixie Geldof grabbed his signature vivid color pieces off the rails of the Regent Street store, he looked on with disbelief. “I was so nervous that no one was going to be here. I’m watching the rails being emptied, literally, it’s freaking me out — in a good way!” For her part, Mischa seemed to agree, congratulating the designer with a huge hug as she arrived in one of his new designs — a studded leather waistcoat and blue tulip skirt. “This is amazing,” she said pointing to a cropped leather jacket, while Sienna Miller, having completed her mission of support, sipped champagne quietly with a few friends. You can finally nab some of Williamson’s must-have looks for yourself. His collection, which ranges from $25 to $349, launchs today in H&M stores. –Reporting by Monique Jessen