In the UK the �age of consent� is 16. You can be charged with having unlawful sexual intercourse with someone under this age. This is so even if the person with whom you had the intercourse gave their consent. (That is, you did not rape them). If they did not give their consent, of course, you would be charged with rape.
The confusion you have concerning the age of 13 is that children under this age are, legally, not able to give their consent at all. So if you had intercourse with somebody aged under 13, EVEN IF THEY GAVE THEIR CONSENT, you would be charged with rape. This is because any consent they give is invalid in the eyes of the law.
So be careful!