But the destruction of the originals IS a smoking gun.
If the two versions were identical, why destroy the originals?
Inquiring minds want to know....
The British Official Secrets Act has teeth and the courts in Old Blighty don't entertain a lot of tolerance for claims by journalist about protecting sources. Under current conditions prosecution may not be politically possible, but with a shift of political winds, it could very likely happen.
What if the original indicated who leaked it...? So you give a copy out that can't be traced. Maybe to protect someone ?