Hi,
Christmas can be very chaotic in London, but you have the lights on Oxford street, Madam Tussaud's wax museum, the London Eye ) I think you will probably need to book this. The Natural history museum.

There are a couple of websites I have found which may be of use

http://www.enjoyengland.com/find-tic/london.aspx

http://www.enjoyengland.com/find-tic/london.aspx

You could always see if there are tickets available for a Pantomime and I think Harrods have a santa's grotto but I am not too sure .

Hope this helps and I hope you have a lovely time.