It's only 'statutory rape' if the victim is under 13 years of age. [Sexual Offences Act 2003, Section 5]
The relevant offence here is 'sexual activity with a child'. [Sexual Offences Act 2003, Section 9]. Where an offender is 18 years of age or older the maximum penalty is 14 years imprisonment. Where (as here) the offender is under 18 the maximum penalty is 5 years imprisonment. [Sexual Offences Act, Section 13].
Whether the CPS would decide to launch a prosecution is difficult to say. In my teaching days (1970s and 80s) quite a few 15yo girls became pregnant (with the identity of the baby's teenage father undisputed) but I can't remember a single prosecution. However attitudes have changed and it's possible that the CPS might decide to charge the offender.
Even so, if the girl was a willing participant (even though she can't legally have given consent), then a custodial sentence would not be considered. That means that, if the male is a first-time offender, the only possible sentence would be a Youth Rehabilitation Order:
http://www.inbrief.co.uk/court-judgements/youth-rehabilitation-order.htm#