Actually, what is quite suprising is the law isn't different. In the sexual offences Act 2003, only a man can rape a child under the age of 13, because its penile penetration only. (section 5)
But any sexual activity with a person under the age of 16 (by someone over the age of 18) is liable, on conviction on indictment, to imprisonment for a term not exceeding 14 years. Section 9 states that if the over-18-person is penetrated (vagina or anus) by a part of the under-18-persons body, they are guilty of an offence. So therefore there is gender mutuality.
This is subject to whether the older person reasonably believes that the younger person is under 16.
Hope that helps
Jason