Withnail and I/WMG

One or more of the characters is a Time Lord.

 * Specifically, Marwood is most definitely the Doctor, or possibly the eighth Doctor's son (Given their family resemblance). This, of course, makes Withnail also a Time Lord by association.
 * There was a cartoon Doctor Who Expanded Universe thing in which Richard E. Grant played the Doctor, with a personality not altogether unlike Withnail's. Also note Withnail's "synous nicotine based" Techno Babble. He must be a Time Lord too.
 * They're actually all the Doctor. The film is taking place in the head of the Eighth Doctor as he lies dying and preparing to regenerate. He meets a possible next incarnation (Grant) and is also attacked by an alternate version of himself (Griffiths was rumoured to be the planned Eighth Doctor if the original BBC series had continued) trying to become real by 'having' him. Not wanting either alternative, he shaves his head, puts on a leather jacket and heads off to Manchester to become Christopher Eccleston.

Marwood is an Unreliable Narrator and the reason there are so few women in the film.

 * Marwood is quite possibly gay. Note how snuggly he is with Withnail, how Have I Mentioned I Am Heterosexual Today? he gets pretty much all the time, and his impressive shabby chic fashion sense. And he's The Narrator and the eponymous "I", so it's fair to say the film is from his perspective. Perhaps he is gay more or less to the point of being a He-Man Woman Hater, or at least women just don't appear on his radar. Since he's in denial about being gay, he's not getting laid, which combines with him being young and attracted to his roommate to make him enormously sexually frustrated, so he therefore doesn't notice women. The only women who have a speaking role are the schoolgirls who Withnail shouts at; Withnail, who is very important to Marwood, is capable of putting women on Marwood's radar by interacting with them.