Could be a baby Royal Enfield, a WD D 250cc (sidevalve single)
They have a small bulge on the primary cover over the engine sprocket, like that.
Forks and mudguards and a few stray other bits look good too.
That footpeg looks like it sticks out a mile ???
Enfields sold quite a lot of them to the military.
They were used for light messaging and all sorts of light duties.
Esp by women in uniform - as here indeed !
A good photo is not so easy to find.
I'll work on it.
This is in military paint - and is the other side,
Can see a lot of the features though\http://gmvg.co.uk/test/wp-content/uploads/9305.jpg
Few RE military projects on the go - slowly !