The V&A, London's palace devoted to all things beautiful, is great all year, but it is especially nice around the holidays. The museum shop is an amazing source of gifts, with a selection of beautiful things that you will not be able to find elsewhere. Prices can range from very fair to crazy expensive, but everything is well selected, interesting, and design-oriented.
Christmas isn't entirely about consumerism (or so I'm told), so the V&A offers holiday activities as well, this year offering workshops on how to "Make a Couture Christmas Decoration". Each year the V&A has an elaborately (and unusually) decorated Christmas tree that is well worth a look.
If you go during the week, you can have the place mostly to yourself, but it can get quite crowded on the weekends. For even more holiday fun, go skating at the ice rink and shop in the little Christmas fair across the street in front of the Natural History Museum.