A) A lot of Britain's military was stationed thousands of miles away throughout the empire whereas Germany had their entire military concentrated in one place.
B) Britain was primarily a naval power and most of its strength that built the empire was in its navy. The Royal Navy would have easily defeated the Kriegsmarine (German navy) but that's not much use in a war on land.
C) Britain is an island, so once France fell it was difficult to keep troops in Europe supplied and re-enforced enough to fight Germany across the sea from Britain.
D) Due to the fact that the main threat to Britain wasn't invasion (the Germans didn't have enough suitable landing craft or control of the English channel) but bombardment and starvation the battle for survival was fought mainly in the air and against the German submarines that targeted supply convoys. This effectivley cancelled out the army and navy and left it up to the smaller Royal air force (who won).
E) Britain's economy relied heavily on exporting industrial goods like steel and textiles to other countries, this is where the immense wealth that built the empire came from. During the war most factories were turned into military factories and couldn't produce regular things to sell. It was also difficult to physically export things to be sold with a war on.
F) Britain's most powerful asset was its navy, while its army was well trained and well equipped but small. The German Blitzkreig tactic involved smashing quickly through the enemy lines with columns of tanks and routing them before they could set up proper defensive positions. The only effective tactic against this was to have lots of space to retreat into and a massive army to wait for the Germans behind the front lines (as Hitler discovered in Russia), neither of which Britain had.
G) In between WW1 and 2 the Germans had been specialising their army for Blitzkreig tactics. This meant that they had well trained soldiers and much superior tanks.
The French assumed WW2 would be trench warfare like WW1 and so had few tanks, aircraft and no reserves behind the front lines to protect Paris. The German tanks just smashed through this and continued unapposed to Paris.
The British (as the inventors of the tank) had tanks, but not nearly as many or as well built as the Germans, however, they had a very well trained army and a similar (if not superior) air force (with aircraft like the Hurricane, Supermarine Spitfire, Halifax and Lancaster bombers) this meant that they had just about the right armaments to counter the Germans, but were not superior enough or prepared enough to defeat them single handedly.