the central top flap is there to assist in sliding open the canopy.
The CFS wisdom had the flap functioning more as a spoiler to break the lift from the canopy allowing it to be opened in flight. As the flap was tiny, I'm inclined to believe this rather than the drag generated assisting opening.
Helmets - At Swinderby, the students mostly wore MK1 helmets. The taller ones just wore the cloth inners. One of my very lanky students was so thin, he slipped out of the harness and his head went through the canopy. After that, he wore the cloth inner only. The instructors mostly wore MK4s which were as bulky as the obsolete MK2s, but had little space problems. I wore a rare MK4b which had a larger visor mechanism allowing individual visor selection. It gave no issues.