In RFU-land, Regulation 15 Annexes 1-7 (Under 7 to Under 14) specify "Rules of Play". So follow what is stated in each Appendix. Only at Under 15 and above are they "Variations to the Laws of the Game". Thus GLDO & 50:22 are only applicable at Under 15+.
Except, you still have to cascade down from WR U19 Laws, you can't follow the "Rules of Play" alone, casing point, a fend, at U13s we have;
The ball carrier may run and dodge potential tacklers but must not fend or hand them off above the armpits.
Nice and clear, the first year that a fend is allowed, sensible progression
at U14s we have ..... nothing, no mention at all so what do we do;
a) No fend allowed, as not specifically mentioned
or (b) Fend as per laws of the game (so head allowed as long as no undue force) - which is how it is at U15 and above (except when refs and coaches do their own thing)
(b) is the correct answer btw
I'm sure there are other examples, but the Rules vs Variations is a nice general distinction