Legal Opinion on GCHQ Surveillance

I am not sure why this is a big deal, my knowledge of the British legal system being less than fulsome, but a British barrister has written a legal opinion calling into question large swaths of surveillance activity by GCHQ, and in Britain, at least, it is a big deal. In the United States, of course, if some members of Congress asked a private law firm to write an opinion evaluating the legality of a government program, it surely wouldn't be front-page news. But the opinion---called an "Advice" and requested by Tom Watson MP, Chair of the All Party Parliamentary Group on Drones---is making big waves in the U.K. Here it is. And here's a blog post summarizing it.