Inddeed. "Threat to the national security" was recently used to quash an investigation into corruption and bribery involved in a deal with Saudi Arabia. Important to have the Saudi royal family on your side, apparently. More important than the law or justice; so the "National Security" card was played and everything got dropped.
This move is about stopping people like Edward Snowden. It what we've come to expect from the Britsh State.
Meanwhile, the government gets up to whatever the hell it likes under the utterly, utterly false boilerplate defence that “all of GCHQ’s work is carried out in accordance with a strict legal and policy framework which ensures that our activities are authorized, necessary and proportionate, and that there is rigorous oversight.”
We live in a country that's little changed from the 18th century in terms of democracy and accountability.