Clues to the Royal Artillery Regiment in the top image
(Also first image
https://s1178.photobucket.com/user/se...nab8v.jpg.html )
The white lanyard.
The leather bandolier of ammunition pouches (artillerymen could not wear standard belt ammunition pouches).

The shoulder insignia (should read RFA (Royal Field Artillery, light guns near the front line) or RGA (Royal Garrison Artillery, heavy long range guns).
If it is possible to scan a high resolution image of the shoulder insignia you may be able to make out the middle letter as an “F” or a “G” which would tell you more of their lives during the war.

The second from the bottom image of a gunner with his hat on the column (also 1st image https://s1178.photobucket.com/user/se...f6jex.jpg.html ) almost certainly shows a member of the RFA as he is wearing spurs (horses were used to move the light guns quickly).

Suggestion the photograph glued into the centre of the cap badge is almost certainly a family commemoration of a son, brother or husband killed in action.
Something very simple to do one simply takes a photograph and cuts it into a circle and glue it onto the badge. It is then possible to take a photo of the composite.
This may even have been worn as a broach at remembrance services or the unveiling of a war memorial.


Cheers
Guy