Just because someone agrees to a level of harm does not mean that this consent has been obtained willingly, or that they are aware fully of what they are consenting to.
The law is quite clear on the issue of real (true) consent, if consent has not been obtained freely (i.e. a defendant has used force) or has been obtained through deception, then this is not consent at all.
R v Tanassum 2000,
R v Olugboja 1982,
R v Dica 2004,