The latest reporting on the Snowden docs by The Guardian shows that the UK's surveillance operation GCHQ was apparently well aware that its activities were almost certainly open to a "legal challenge" and therefore they were committed to keeping them secret to avoid such a challenge. Note that this is quite different than the official excuse always given about being worried about public disclosure putting national security at risk by revealing "sources and methods." Instead, here it seems clear that the secrecy was for the very reason that many of suspected it: they were pretty sure they're breaking the law, or at least coming so close that it was something the courts would eventually have to decide... but only if the info got out. And, it wasn't just them. They realized that the telcos willingness in passing on info likely opened up other legal challenges as well.
GCHQ lobbied furiously to keep secret the fact that telecoms firms had gone "well beyond" what they were legally required to do to help intelligence agencies' mass interception of communications, both in the UK and overseas.
GCHQ feared a legal challenge under the right to privacy...