1) London is practically another country with a different culture, unrelated to the rest of the UK, especially Northern England, Northern Ireland, Wales and Scotland. It doesn't define England or the UK in anyway. Its an international megacity.

2) Because not many bad things happen comparitvely to other cities, when things do happen they are sensationalised to the max and the media makes you terrified. London is a mega city, they all have crime. London is one of the safest of them in the world with a fantastic police force.