Originally Posted by
MSPeconomist
In London, public transit closes down fro Christmas.
If you arrive before Christmas, the Vienna Christmas market would be special.
Since it sounds like it may be relevant for the OP, exact timings may be important.
Public transit in London closes down on Christmas eve starting around 8:00pm, so unless you are planning to be in London after 6:00pm on Christmas eve, you won't have any problems.
As far as Christmas markets in Vienna are concerned, some close on the 23rd, but a couple are open till the 26th, and one stays open till the 31st.