'"england can never be conquered" um yeah tell that to the celts and the saxons.' - I believe that England did not exist as a single country when they invaded. Of course, the Normans did take advantage of Harold II's battle-weary army to conquer what was at that stage definitely a united England. And the Vikings certainly conquered England. But, since 1066, no invading army has set foot on English soil. Britain did not exist before 1707, when the Act of Union was passed (Scotland and England were separate entities before then).
Going to the actual question, Germany was remarkably close to taking Britain put of the equation. However, there was one thing that stopped them: Hitler. He was of the belief that Britons were Aryan, too, so he did not dislike them. He even offered peace to Churchill, who refused. Sadly, as he discussed in Mein Kampf, he had an extreme dislike of Slavic peoples, meaning that his desire to take out the Soviet Union was greater than the rationale of neutralising Britain first.
Obviously we can never be sure what would have happened had he been persuaded the other way, but I have a suspicion that the US would not have intervened in Britain's favour (or at all for that matter) as I agree with cgwhite above in that I don't think the Japanese would have attacked the US. I wouldn't like to predict what would have happened when Germany did eventually invade the Soviet Union, though!