There is no need to be afraid when traveling to London, but it is also smart to take precautions. In such a large city as London there will always be criminals making life harder for the nice guys (like us). Therefore you should always pay attention to your valuables, especially on the public transportation and near famous attractions.
According to the EU London has the worst security among the members of the European Union. But still there is no need to be afraid, but take care and use common sense!