http://en.wikipedia.org/wiki/Rape_in_English_law

From the Sexual offenses act of 2003

1-(1) A person (A) commits an offence if—

(a) he intentionally penetrates the vagina, anus or mouth of another person (B) with his penis, [emphasis all mine]

(b) B does not consent to the penetration, and

(c) A does not reasonably believe that B consents.

(2) Whether a belief is reasonable is to be determined having regard to all the circumstances, including any steps A has taken to ascertain whether B consents.

(3) Sections 75 and 76 apply to an offence under this section.

(4) A person guilty of an offence under this section is liable, on conviction on indictment, to imprisonment for life.