Britain in WW2

Source of Map: Camping, Maps, & Travel

The question was:

The British had the largest empire and navy in the world at the time. so why did it take them 4 years including the help of their allies to defeat Germany?

It depends how you look at it. Who really had what?

Germany under Hitler had the following when it started out:

The German Peoples of Germany and Austria: This was a united Nation of disciplined, intelligent, fanatical dedicated warriors, who outnumbered and out-martialled (i.e. were more soldier-like) everybody else.
 The de facto support of the Roman Catholic Church. This changed later but here we are discussing the starting point.
The active support of Italy, Spain (Franco offered to join the Axis only later changed his mind), Romania, Hungary, Bulgaria, Croatia, the Arab world.
The de facto acceptance of nations such as Sweden and Switzerland and Finland.
The full co-operation of Russia under Stalin, along with that of International Communism. They WANTED to work with Germany and in the beginning did so. That the Germans did not want them is another story.
Turkey and the Muslim World would have linked up with Germany once German forces reached their region. 
The automatic alliance of International Jew-hatred. Instead of talking about the Elders of Zion someone should start examining the Union of Edomites i.e. the International Jew Hatred Alliance. This earned Germany the support of anti-Semites everywhere. This was one of the factors that weakened France from the very beginning. You may say that this is not rational but take a look today at the number of nations in the UN who consistently vote against Israel. Hatred of the Jews (whether rational or not) is a factor in world affairs.
A strong pro-German body of supporters in the USA. It was only due to the machinations of Churchill and Roosevelt that brought the USA over to the side of Britain.
Most nations of Latin America were more pro-German than anything else. 
The close alliance of Japan in the east whose starting position and initial successes were quite decisive.
Nazi Germany may have been racist in its ideology but in practical terms non-Aryans peoples seem to have been quite sympathetic towards it. If anything they too viewed the Germans as somehow superior.

What did Britain really have, at the beginning?

A little island, a few colonies, and an affinity with the Dominions of Australia, New Zealand, Canada, and South Africa. It also had India some of whose intellectuals and leading politicians were sympathetic towards Germany.
Britain was suffering from class divisions, economic backwardness, a small army, and a little airforce whose best planes had been provided by private philanthropists and public charity. Many of its working classes were under-educated and underfed. The last thing most of the British people at that time wanted was a fight-to-the-death struggle with the greatest power in Europe. In fact Germany at the beginning represented most of Europe. Even Poland may have joined Germany if the Germans would have been polite about it. The British had the support of underground pro-British forces in Europe and especially Norway. Norwegian support helped the British cripple the German navy at the very beginning, prevented the Germans from making an atom bomb, and kept German troops (equivalent in number to those lost at Stalingrad) permanently incapacitated by having to occupy it. Compared with the initial advantages that Germany did have British chances did not look that promising, certainly not certain.
What other advantages did Britain have?
British, Jewish, and other scientists.
Geographical advantages in being an island.
A reservoir of good will in the USA, and elsewhere (including Ireland) that was used to advantage.
A better and bigger navy. 
Pilots and Air Force leaders who were better than their German equivalents.
 A superior Intelligence Arrangement of forces.
An attitude of "Playing the Game."
Nevertheless, instead of asking, 'why did it take Britain 4 years including the help of their allies to defeat Germany'? one should perhaps inquire,
'How did they manage to do it in such a short time?'