Rose Tyler: Don’t you think we should run?

Doctor: Fascinating history, Downing Street. 2,000 years ago, this was marshland. 1730, it was occupied by a Mr Chicken. He was a nice man. 1796, this was the Cabinet Room. If the Cabinet’s in session and in danger, these are the four safest walls in Britain. End of lesson. Installed in 1991. Three inches of steel lining every single wall. They’ll never get in.

Rose Tyler: And how do we get out?

Doctor: Ah.

From Doctor Who – Season 1 Episode 5: ‘World War Three’ (1×05)