Historical version 4 of St Paul's Cathedral (view current version)
St Paul's Cathedral, at the top of Ludgate hill is a fabulous building. It is a masterpiece of Sir Christopher Wren, Finished in 1710 after being rebuilt after the fire of London it is a symbol recognised as London the world over.
The building is obviously a cathedral and services are held regularly.
You can visit and find famous dead people!
The climb to the top is worthwhile, stopping at the Whispering gallery and later coming outside. There are steps all the way to the golden ball at the top of the dome, though I do not believe that the golden ball is open to the public any longer.
OS X co-ord:
532067
OS Y co-ord:
181148
(Latitude:
51.513318
Longitude:
-0.096550)