Turn right at the Houses of Parliament, walk along Whitehall until you get to Horse Guards Parade.
On the opposite side of the road is Banqueting House, the only remaining part of the Whitehall Palace. Designed in 1619 by Inigo Jones.
Here, for six pounds you can lie on a beanbag and stare at the ceiling for your whole lunch break.
Painted around 1636, by the Flemish painter, Sir Paul Rubens, now remains his only in-situ ceiling paining. After a mix up on the measurements (length of a foot is different in Belgium) it was cut to size by Inigo and Rubens’ apprentice. The artist never did see his work.
You also get an audio guide.
Yes it really is opposite Horse Guards Parade.