Date is WW1 post 1916 - some are wearing the economy tunic, no pocket pleats or shoulder patches.
Regiment - tempted to say none of the above. The badge is not entirely circular, the sides are near vertical. The Gloucestershire Regiment was the Sphinx atop a scroll "Egypt" and 1st Life Guards was a small circle inscribed Life Guards with a crown atop so not those two. There is a resemblance to the Dufferin Rifles badge which has straightish sides with a 38 in the centre.
However, the uniform is not the seven button, stand up collar, no button on the lower pocket flaps Canadian uniform so I pass on a positive ident.
MaxD