There are two versions of Oh No It Isn't! This one is a novel by Paul Cornell, part of Virgin's New Adventures series of novels. (The other is an adaptation of the novel into an abridged, full-cast audio drama by Jacqueline Rayner (though usually credited to Cornell) for Big Finish's Professor Bernice Summerfield series.) These are different works in different media, and they should not be combined.
Professor Bernice Summerfield's afterlife is proving as inexplicable as her life had been. After the ship she's on is attacked by a Grel ship, Benny wakes up in a demented fairy-tale world. Other people on the spaceship--her students, members of the crew, even her cat Wolsey--are also present, if transformed into strange fairy-tale versions of themselves; so are the very confused Grel warriors.