Pic 2 is P37 battledress and MkII helmets - both came out in 1937 - so I would wager later than that. Webbing is Pattern 08, not Pattern 37 which is odd. Most likely a boot camp pic as they are missing patches and gaiters - and they look quite green as none of them wear their gear the same. If I had to guess I would say 1939 volunteers for the war posing for a pic soon after they were issued their rifles and old style webbing. New webbing, gaiters, patches and rifles were issued when they graduated and assigned units. Just an educated guess.
1st and 3rd pic are of sailors which I am not familiar with, but all have P08 webbing as well.
What I find strange is all the SMLE's pictured have sling swivels attached. I thought they were removed after being declared obsolescant in 1914. Perhaps they were put back on for training so they could stack their rifles?