Here is the simple explanation for consent while under the influence of alcohol. R v Bree 2007.
The Court of Appeal held that
“If, through drink (or for any other reason) the complainant has temporarily lost her capacity to choose whether to have intercourse on the relevant occasion, she is not consenting… However, where the complainant has voluntarily consumed even substantial quantities of alcohol, but nevertheless remains capable of choosing whether or not to have intercourse, and in drink agrees to do so, this would not be rape.”