Militart police did not have Ptes- the minimum rank was LCpl. I think he has a set of Cpl's stripes on his right arm only.
That is assuming we are seeing him in the RMP in the pic. The cap badge appears to be the wrong sort as far as one can tell from such a small pic. The RMP badge is an open scroll-work. However the puttees and belt appear to be white, not sand-coloured - and that might again lead to the police.
Wiki tells me that At the beginning of the Second World War, the CMP had several branches:
Special Investigation Branch (SIB);
Red Caps, who were responsible for general policing;
Blue Caps (Vulnerable Points), responsible for security of static locations and establishments;
White Caps (Traffic Control); and
Field Security Wing (Green Caps), which was separated from the CMP in 1940 to form the Intelligence Corps, and who wore the CMP cap badge, but without the scroll.
Our man has a cap with no coloured top.