What establishments, if any, serve cold imported German beer
(preferrably on tap) in London, UK.  Koening Ludwig Dunkel especially.

The official website of "König Ludwig Dunkel":

http://www.kaltenberg.de/koenig_ludwig.asp?lang=en

http://www.kaltenberg.de/

And if you click on International there, it opens a map.  A click on
the UK opens a page telling that since 1979 their beers are brewed
under license by the Daniel Thwaite Brewery:

www.thwaites.co.uk

Under Pubs you can find the one nearest you and call to ask if they
have what you want, or call the main office.

Prost!

The map mentioned by myoarin-ga will open "if you click on
International" and then on "The countries with beer of Royal
Highness":

http://www.kaltenberg.de/kli/laender_en.asp

http://www.germanbeerguide.co.uk/london.html is a great link for
finding pubs in London and it lists German beers on tap in London
pubs. I have to say it might be easier to find this beer König Ludwig
Dunkel in bottled form than it is on tap. I did the Thwaites pub
locater search as the previous poster suggested and found the nearest
pub that could have that beer was 94.1 miles from London. The Thwaites
brewery is in Lancashire which is half a day away from London.
To get this particular beer, I would search the beer markets listed in
the link above. Otherwise, the link lists a lot of London pubs with
other German beers on tap. Enjoy!

As an old "Bier" drinker, living where the stuff is made, I wouldn't
get too hung up on the stuff Prinz Luitpold brews, and then by license
by Thwaites.
I am sure it has a certain social "cachet" to know you can order the
brew, but does it really taste better?
Now when I am in the UK, I look around and see what's served in the
glass that I need for my collection and order that.  Pint of Young's
the last one.  ;-)