The United Kingdom has the most powerful military in Europe for a variety of reasons. First off, England's army has more combat experience than any other in Europe. As far as man power, only Turkey has as many soldiers in Europe as the United Kingdom and the quality of them is dubious. Germany and France each have a little more than half as many active duty personnel as the United Kingdom. And in the case of Germany a great deal are conscripts instead of regular soldiers. The United Kingdom has a modern MBT arm and the RAF is one of the most powerful air forces in Europe.
The area where the United Kingdom far out does any other European power is with its navy. The UK has 4 carrier, to France's one carrier (although it is new). Germany does not have any carriers.