Well, the most popular Christmas activity in London is the famous Christmas lights display along Oxford Street. You'll also find various Christmas markets and carnivals throughout the city, where you can enjoy delicious food and drinks, and participate in fun activities. And don't forget to take a picture with Father Christmas himself at Harrods! For those seeking a more traditional holiday experience, you can attend a Christmas service at one of London's many beautiful churches. Or you could simply spend the day exploring the city's many museums and galleries. Whatever your interests, London has something to offer!