The party took place at Oswald’s, an exclusive private member’s club owned by Robin Birley in the Mayfair neighborhood of London. Mr. Birley, London’s king of clubs, named Oswald’s after his grandfather, whose portrait hangs behind the reception desk. A 50th birthday is a major milestone in any woman’s life — though not everyone has to step into it like Victoria Beckham, who not long ago broke her foot in a gym accident. Still, Mrs. Beckham was determined to celebrate in style in London on Saturday, when, at least for one night, she replaced the medical boot she has been wearing for months with pointed toe stilettos, visible panties and a long sheer sea-foam-green gown from her own label.