The Titanic was a spaceship replicated by aliens from the original Titanic for tourism. It is owned by Max Capricorn and was captained by Captain Hardaker. It used Hosts as information and service robots.
The ship came under attack by three asteroids, then later it fell to Earth, it very nearly hit Buckingham Palace however the Doctor was able to restart its engines returning it to orbit. (DW: Voyage of the Damned)
The Titanic crashed into Buckingham Palace and destroyed London when the Doctor died in an alternate timeline. (DW: Turn Left)