What a great present! You could find a small bunch of London souvenirs (I don't know if I can post a link, but you could search on ebay for Cute British Miniature London Model Key Ring and find matches) and attach a clue to the next clue, and make it a treasure hunt of sorts. The last package would be the tickets!

Are you fans of something specific that is British, such as Dr. Who, Downton Abbey, Shakespeare? Then you can be more specific. If you didn't want to buy little items (such as those above) you could print out images of places where you are planning to go onto card stock, and write clues to the next clue inside.

Will you be spending Christmas Dinner together? You could host a truly British style Christmas dinner.

You will have a great time!