The very last files that the UK National Archives plans to digitalize and put on line won't be the ultra-secret Ultra files . Trust me : Ultra was a success.
No the very last files, if ever, to be digitalized and put on line will be the series FO (Foreign Office) 370/926 : requests from aboard , 1944-1945, for that wonderful British invention , penicillin.
That's because what should have been the greatest success for the UK in WWII turned out to be their worst total balls-up disaster.