The war diaries are themselves interesting as hand written records of the daily actions of units in the front line. The idea is to get us to do the classification and many people have clearly already been taking part.
There is a much more on the site including access to a lot of other documents. I was searching for materials on the Salonica campaign and came across this wonderful memo from the First Sea Lord, Jellicoe telling the War Cabinet, in gloriously pompous tones, of the need to withdraw troops in May 1917. Somehow the typewriter gives the words more feeling than seeing them reproduced in a book. You can almost hear Jellicoe dictating them to a secretary.
There are also General Milne's (He commanded the British Army) regular reports of his and the allies actions to the War Office.
There are also podcasts and film clips. I particularly liked this one of Franco's attack on Barcelona in 1939. The artillery far from being spaced out in WW2 style is firing from a column formation.
The National Archives are certainly providing a great resource.