Is there another definition of penis?

It does clarify that

3)References to a part of the body include references to a part surgically constructed (in particular, through gender reassignment surgery).

So a trans man with a surgically crafted penis would count as rape.

Anything else would be assault by penetration which is

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

(a)he intentionally penetrates the vagina or anus of another person (B) with a part of his body or anything else,

(b)the penetration is sexual,

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

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

A woman can commit assault by penetration if she were to digitally penetrate someone else, but not rape. Only penetration with a penis.

There’s also sexual assault which is

1)A person (A) commits an offence if—

(a)he intentionally touches another person (B),

(b)the touching is sexual,

(c)B does not consent to the touching, and

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

In all of these “he” no longer means male only though. So you can replace “he” with “they”.

Rape still requires a penis, meaning only a male, trans woman prior to bottom surgery, and trans man after bottom surgery to create a penis can commit rape under UK law. Or some edge case that I’m sure exists in single digit numbers, where a woman that identifies as a female gets bottom surgery to get a penis.

Any gender can commit sexual assault or assault by penetration though, which carry the same penalty as rape.