Junkyard you are pretty much right.
If the child is under 13 the offence of rape is committed, whether or not they consent (more often termed statutory rape, mandatory rape could mean something entirely different) – Section 5 SOA 2003. The rationale being that a child under 13 cannot give informed consent.
If the child is 13,14 or 15 years and consented, the offence is sexual activity with a child. This is either Section 9 or Section 13 of the SOA 2003, depending on the age of the offender (18+ and under 18 respectively), with lower penalties and greater discretion over whether to prosecute when the offender is under 18 as you point out.
If the child is 13,14 or 15 and does not consent obviously that is normal rape.
EDIT This is for England and Wales, Scotland’s got different laws, although I think a Scottish version of the SOA is on it’s way.