Well something is not adding up here.
The CPS Sentencing Guidelines state that:
Rape is a serious specified offence to which dangerous offender provisions apply (part 2, Schedule 15 Criminal Justice Act 2003)
The starting points in the guidelines are a) for offenders who do not meet the dangerous offender criteria and b) as the basis for the setting of a minimum term within an indeterminate sentence for those who do meet the criteria.
Starting Points
Single offence of rape by single offender: 8 years custody - victim 13 or over but under 16
Rape accompanied by aggravating factor: 10 years custody- victim aged 13 or over but under 16
(NB. Abuse of trust is classed as an "aggravating factor")
www.cps.gov.uk/legal-guidance/rape-and-sexual-offences-chapter-19-sentencing