Wait a minute - I missed this, from the 2003 Act:


13 Child sex offences committed by children or young persons (1) A person under 18 commits an offence if he does anything which would be an offence under any of sections 9 to 12 if he were aged 18.
(2) A person guilty of an offence under this section is liable—
(a) on summary conviction, to imprisonment for a term not exceeding 6 months or a fine not exceeding the statutory maximum or both;
(b) on conviction on indictment, to imprisonment for a term not exceeding 5 years.


So there it is. Sections 9 to 12 cover pretty well all the usual types of sexual activity - therefore it is an offence for any person under 18 (or over) to have any kind of sexual activity with a person under 16 .... Now we know.