Consent and children comes up on these threads a lot. I looked it up:
If you have sex with someone aged 13-15 who is willing to have sex with you, then you are guilty of unlawful sex with a minor, but not rape. So the consent is taken into account and makes a big difference to what you are charged with and the sentence you get.
If you have sex with a child under 13 then, consent or not, you are charged with a specific offence of unlawful sex with a child under 13 and in very deep trouble. The fact the child was willing might be some mitigation (assuming anyone believes it) but you're up shit creek and the sentence will be very long anyway.
I think the courts start with an assumption that the younger the child, the less likely it is that consent (as any sane adult would understand it) could possibly be present.
We do not have any offence of statutory rape in the UK.