Failed Head Butt

OB..

, Advises in England
#41
that's essentially true (I think it's a bit trickier and more subtle than the way you expressed it) - but that's because it's the way the Law works - it's not because the RFU have decreed it to be so.
The defence is "volenti non fit injuria", which clearly applies to boxing, for example. However it has been rejected in a number of rugby injury cases because the cause of the injury was not deemed to be a normal part of playing rugby.

Danny Hearn broke his neck trying to tackle Ian Macrae, but everyone agreed it was just a very sad accident (the two are actually good friends these days). However in the cases of Ben Smoldon and Richard Vowles, the broken neck was deemed to be the referee's fault because he allowed unsafe scrummaging.