From the TfL website:

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We have not operated any Tubes on Christmas Day since 1982. There is
very little demand for services as the majority of Londoners do not
work on Christmas Day and over 70% of London's population leave the
capital during the festive period. Limited bus services operate
to/from the major airports and in central London. Please call Travel
Information on 020 7222 1234 to find out more.
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70% ?? Can this really be true? So London's population (being ~8
million depending how you count) drops to ~2.4 million at Christmas?
Surely this cannot be true.