Here is a really cool and exciting way to have a bird's eye view of London-by taking a helicopter ride from one the many tour companies based either Biggin Hill or Redhill airports.
Some friends and I did that a few months ago when it ws really and had some real fun flying 1400 feet over the centre of London and seeing the Houses of Parliament,Buckingham Palace,the Tower of London and many more amazing sites.

The prices of the rides do vary depending on the type of vehicle you hire the higher the flying power the more it costs and prices start at around 119.00 for the most basic. Do remember that if you are going with friends that you can only take 3 because you need to have room for the pilot. Take to call and get some safety advice, because there limitations on what you can or cannot do.