The first one is deffo Boys Brigade

The next are all WW1
The second is Royal Artillery, but not mounted as his puttees are wound bottom to top, not top to bottom as a mounted trooper would wear

The third is Royal Engineers, and on that one he is wearing his puttees as mounted!!!

This pic is post 1918 as he is wearing overseas chevrons on his right bottom sleeve, and he is a signaller as per the arm band and crossed flags above his Cpls chevrons...