Yep that is a SLI!....
Shoulder pads, pleats and stiff cap...Early WW1. 1914/1915
He also has a walking out cane...Boots are worn in and a bit mucky, but look at the creases in his jacket sleeves and trousers! Smart as carrots!
His puttees are a bit of a mess though! Not very tidy at all!!! How did he get away with them?? the pointed ends are really uneven...
I would have put him on a charge for them..And for "dust in his welts"

Quite odd given the state of his jkt and trousers, his boots and puttees are somewhat slovenly
My gt uncle joined the SLI, he went down from Northamptonshire/Bucks border and joined them as well...So I wouldn't worry too much about a Zummerset connection...