Basically, the BBC is revered and well-respected because they do a great job of news coverage and don't slip into the easy "soft news" stories like so many other news organizations do. It's more about their consistent and reliable body of work over decades than anything else. But, as jack020 points out, they took quite a credibility hit recently:
http://www.cnn.com/2012/10/23/world/...dal/index.html