It could be argued that Audrey II should be eaten for reasons of poetic revenge, and although she is of course a strange and interesting new plant, I have some sympathy for that idea. It does potentially provide a tiny food chain consisting of two species as well, which is quite interesting. Nonetheless, Audrey II is a sentient life form. I do think it would be OK to eat her if she had been killed in self-defence, which I seem to remember did happen, although she might be poisonous. This reminds me of the idea of eating road kill. It would be a shame to eat an unusual species of plant unknown to botany without analysing it a bit.
The first reason for not eating Audrey is naturally that she is clearly sentient. She uses language. That's not what makes her sentient even if Christine Battersby says it is, but it is evidence of her sentience. She is able to report on her own mental events. A less "intelligent" but still sentient species of fictional plant is of course the triffid. In the case of both the triffid and Audrey II, it would be wrong to kill although it would probably be a good idea to give both a wide berth in the same way as one might a tiger. No reason actually to kill them as such though, because the world was not put here for mere human convenience and we have a duty to live and let live if at all feasible.
The second reason also applies to real carnivorous plants such as the Venus Flytrap
"Venus Flytrap showing trigger hairs" by Picasa 2.0. Licensed under CC BY-SA 2.5 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Venus_Flytrap_showing_trigger_hairs.jpg#mediaviewer/File:Venus_Flytrap_showing_trigger_hairs.jpg
and certain bromeliads, although it's quite convoluted. Sleepoversweet once maintained that if it was OK to eat any kind of animal, the best ones to eat would be carnivores because that way their prey's lives would be saved. I find her argument utterly mind-mangling and fascinating, although I don't agree with her. By the same token, maybe a vegetarian ought to base their diet solely on carnivorous plants because that way she'd be saving their prey. Some bromeliads, namely pineapples, are edible but they tend to try to eat you at the same time as you try to eat them, at least in Daniel's case.
This is where it gets confusing because of the food chain. One of the arguments for veganism is that you put less strain on the biosphere by eating only organisms which make their own food and don't move around much. This means you're eating almost as efficiently as possible, and in fact you'd probably be eating more efficiently than if you ate only food which was entirely synthesised from non-biological materials because on the whole, living things are amazingly efficient at what they do, probably as efficient as possible given the materials at their disposal. Even so, there is a bit of a quandary here because everything gets recycled, so if a herbivore uses of the order of ten times the quantity of plants to produce the same mass of material and the carnivore that eats it multiplies it by another factor of ten or so, how come this doesn't spiral out of control when the dead bodies and waste of these animals or plants are eaten by the detritovores and saprophytes such as houseflies and mushrooms? I think the answer must be that they don't move much, but to be honest I don't really understand it. After all, a housefly seems to move quite fast to me (and incidentally does it make sense to think of flying insects as "cold-blooded"?).
Pineapples and their kin don't move around much though, so they are not inefficient compared to antelopes in producing food for other organisms. This is the reason why it might be OK to eat Audrey II, particular considering that she seemed to use a heck of a lot of energy compared to most plants. Nevertheless I have to admit to being quite confused now. I think it's probably a bad idea to eat carnivorous plants because they occupy the same tropic level as some carnivorous animals, but by that same argument it might be a bad idea to eat any fungus which has consumed an animal.
Anyway, here's a picture of the Doctor pulling Sarah Jane Smith out of a conduit: