what were the causes of the fall of the british empire?

One modern perspective is John Keegan's book, "The Price of Admiralty" --
http://www.amazon.com/gp/product/0140096507/103-3307214-3567060?v=glance&n=283155

First, the Decline of the 'core' businesses on which the British
Empire had been founded: Slave Trade; Drug Running; Piracy ...

Second, the Invention of the Internal Combustion Engine and the
attendant rush to discover oil.

Third, the Jealousy of other countries (Japan, Germany, Russia, USA)
who quite unreasonably also wanted a piece of the action.

Fourth, the Loss of England's cricket supremacy and the symbolic
burning of The Ashes.

Fifth, Humiliation in the current Test Series against India.

Sixth, Giving Women the Vote.

Seventh, Making a Woman the Prime Minister.

Eighth, the Death of Princess Diana.

Ninth, a lack of Appreciation by former Colonies (particularly the
American ones) who wrongly perceived that they could do things better
themselves, eventually culminating in the emergence of George W Bush.

Tenth, the Madness of King George.

Phew!

That's quite enough.

...eleventh: Porridge.

I was once taught (probably in a random history or economics class in
college) that empires fall due to conquest or a general unhappiness
within the middle class.
We can see that Britain was not conquered.  Most of the British empire
around the world did indeed have a very unhappy middle class.  These
are people in various areas who either had a lifestyle they were
accostomed to before the British arrived or who were British and left
Britain in hopes for something better... in either case, for the most
part they were not made happier by British influence in their lives.

12th ---> Spice Girls!!

13th... the death of george best... who may have been irish... but what the he..

How about the death of Paul McCartney? And the need to replace him
with a double? And then Yoko comes in?

These were cetainly contributing factors.

two ruinous wars with germany.  america just picked up the pieces
after world war 2 and became the new boss.

Spread of Education by British Empire.