Here are the public transport arrangements in London for the holidays. One thing to note is that Black Cab taxis (if you can find one on Christmas Day!) are allowed to add a £4 surcharge to all fares for journeys from;
8pm on 24th Dec to 6am on 27th Dec and
8pm on 31st Dec to 6am on 2nd Jan
http://www.tfl.gov.uk/assets/downloads/Christmas_travel_2009.pdf