Consequences are for the proles
We've long known this - whilst the law may apply to the rich, powerful, ministers, prime ministers, spies etc... What actually matters is the consequences.
The reality is that the organisation that would bring any prosecution is controlled by the government - the CPS. They have absolute control over this agency, as part of the executive. If something is determined to be covered by "national security", at the sole view of the Attorney General, who is appointed by the government of the time, then it is entirely up to them whether it goes ahead.
Well, unless someone pays for a private prosecution that is... Ha.
With a majority like the one Boris Johnson has now also, it would be very difficult for anything to stop a change in the law also. So, the Supreme Court's powers are still limited by the government.