He does it because he is the Professor for the Public Understanding of Science at Oxford, which means that it is his job to ensure that the public is actually receiving a scientific education ie. not based on religion.
His main fight on religious grounds is with America, not the UK. He does this, thankfully, because there are people who think that we should be teaching our children that the world was created 6,000 years ago. Thank goodness someone with a platform has the ability to speak out against the religious figureheads in America, because without them Americans could still be thinking that Adam & Eve is a true story and that dinosaurs are purely ficitonal.
Whether you like him or not (I think he's excellent and hilarious - echo that comment about him and Hitchens kicking people's arses on youtube), he has a very important job to do and he does it very well.