> In the UK a sixteen year old is considered to be old enough to give consent in a sexual relationship.
Not quite. It's 16 unless with someone in a position of trust, e.g. your school teacher, social worker, doctor or care worker. A person under 18 cannot legally consent to that in UK.
> A person under 18 cannot legally consent to that
That's a bizarre way of putting it, isn't it? It's not as if the person under 18 would be committing an offence by giving consent, and the consent would still be valid for defending against an accusation of rape (as far as I know; see the Sexual Offences Act 2003).
