Rather than doing interpretative dance to reach over the cheek-piece and down the side while the weapon's tilted to the right, you reached across under your right forearm to come at the cocking handle from below?
I can't see why that'd be a problem, either.
What you've shown there is an L98A1, though, isn't it? The L85A1 has a stubby cylinder sticking out of the bolt carrier where the back end of the long black arm of that thing's cocking handle is, and is cocked left-handed. That thing's cocked right-handed using a handle a lot further forwards, in front of the breech rather than behind it.
I think the reason they want you to cock it overarm is that you're supposed to take the handle all the way back and then keep your hand moving backwards so your fingers roll off the handle, much like the draw and loose action used with a traditional bow. If you pull it back underarm, you're likely to ride the handle forwards and sometimes it doesn't go all the way if it's impeded. Even when it's clean, you can rest it just before the end of its travel. If you've just fired a magazine of blanks at the Sergeant's Course, you're likely to have to shove the bolt the last quarter of the way forwards through all the soot.
