Somebody who most likely has signed the official secrets act, leaking confidential communications for political purposes. I would have there was at least a case for a treason charge. Last person to be convicted of treason was William Joyce, who was broadcasting support for Nazi Germany. Presumably the clause in the treason act of 1351 under which he was convicted was this:

"Levying war against the king in his realm, or adhering to the king's enemies, giving them aid and comfort in his realm or elsewhere"

To convict the leaker you would have to make the case that aid and comfort were given to the enemies of the sovereign by the leak. Which sort of depends upon who you think they are.