P4X is a reference to the P4W (Prison for Women) in Kingston, Ontario, Canada which was the only women's maximum security prison in Canada before it closed in 1997.
(In the prison)
Heidi: Xev what?
Xev: Xev Bellringer of B3K.
Heidi: Is B3K in Texas?
Heidi: Then where is it?
Xev: B3K is another planet, far away from here.
Heidi: Ain't no space on this form for other planets.
Xev: All I know is that I'm from the planet B3K. I don't know where on it exactly.
Heidi: OK, let's make this easy. Did you grow up in a city, town, or in the countryside?
Xev: I grew up in a box.
Heidi: That'd be Houston then.
Craig Charles and Hattie Hayridge (who play Lister and the female version of Holly on RED DWARF) appear in this episode as prison wardens, but strangely they are not credited as guest stars at the beginning of the episode, only at the end.