A London institution since the 1940s, Pandora stands in fashionable Knightsbridge, a stone's throw from Harrods. Several times a week, chauffeurs drive up with bundles packed anonymously by England's gentry. One woman voted best-dressed at the Ascot Horse Races several years ago was wearing a secondhand dress acquired here. Prices are generally one-third to one-half the retail value. Chanel and Anne Klein are among the designers represented. Outfits are usually no more than two seasons old.