Under the old law , there was a curiously worded defence viz that a young man of this age who had not been previously charged with a like offence and who had sexual intercourse with a girl of this age was entitled to be acquitted if he did not know that she was under age. (Curious because it was not obvious why his never having been charged made any difference; it did not say never having been convicted)
Judging from the Crown Prosecution Service notes to the new Act, the Sexual Offences Act 2003, he is entitled to be acquitted if he reasonably believed that she was only 15 .In any event, the CPS are not at all likely to recommend prosecution . it is not as though he was in some situation where he was in breach of some special trust (like, say, a teacher), where different considerations apply.
Their both being drunk or under the influence of cannabis is, unusually, not particularly relevant. Nobody is suggesting that she was raped. It is a case of underage sex; she is 'below the age of consent' and the offence is in that,however 'consenting' an underage girl is, or why.