Diptels are official secrets for a good reason. They allow our ambassadors to write freely, openly and without constraint to our government, informing our complex web of foreign policy and furthering the interests of the United Kingdom. Ambassadors are not hauled before select committees to justify their views, or subjected to deep personal scrutiny by the mainstream media. In return they keep a low profile, don't Tweet, don't snort lines of marching powder at nightclubs and generally act .. diplomatically.
Leaking Diptels is neither in the interests of the nation or even of Brexit. Once you start to make exceptions based on the degree of commitment of a leaker to their cause you open a Pandora's box. A Polaris submariner who opposes nuclear weapons? An SIS field operative who abjures subterfuge? A Home Office staffer who disagrees with telephone taps on Islamists? Exposing illegal State activity is one thing - but leaking perfectly proper, legitimate, lawful Diptels is not whistle blowing. It is a breach both of trust and of the criminal law.
Warning newspapers in advance, as the Met have done, that if they publish more stolen Diptels that they will be guilty of criminal complicity is also right. The MoS is not publishing these from the good of its heart but because it increases sales and profits for its owners. The Mail was quick enough to drop Brexit like a hot coal when their global corporate advertisers put the squeeze on - they are hardly motivated by virtue or altruism.
Freedom of expression and freedom of the press is not the issue here. The system that helps preserve our national security is.