I know that, but it's my understanding that if a victim is too drunk to consent, she's too drunk to consent. Period.(?)
I prefer to put it this way: It's rape if the victim does not consent
and the shagger (a neutral legal term

), does not reasonably believe that the shagee is consenting.
So: Assume the jury are sure the shagee was so intoxicated as to lack capacity to consent. Consent is a positive state of affairs, not the default position, thus "can't consent" = "doesn't consent".
End of the matter? No.
The shagger says a) "the shagee consented"; b) "even if you the jury are against me on that, I genuinely believed that she consented". The jury, in order to convict must now be sure that the shagger had no reasonable belief in consent. That exercise necessarily involves:
"Are we sure she did not consent - Yes, she could not so she did not"
"Are we sure that the defendant did not have a genuine belief that she had the capacity to consent (we're sure she was so intoxicated as to lack capacity, but, in all the circumstances prevailing at the time, might the defendant have reasonably believed otherwise?) If sure no such reasonable belief - Guilty. If not sure:
"Are we sure, having concluded that he might have reasonably believed that
she had the capacity to consent that the shagger had no reasonable belief that she did
actually consent.
In short, to convict, in a case like this: A unanimous decision (we're all sure) that:
1. too intoxicated to consent, ergo did not consent.
2. no reasonable belief she was able to consent. Guilty verdict. OR:
Not sure of 1 - Not Guilty.
Sure of 1 but not sure of 2 - consider 3
3. no reasonable belief she actually consented - Guilty. Not sure - Not Guilty.
Phew!