I know you can get frozen fish to feed but other than that it's implied to be illegal to feed a live vertibrate animal to another animal. It is however not particularly inforced and the legislation about it is not specific and as of yet(from what I can find) no case has been taken to court and therefore the courts have not interpretted the legistlation one way or the other.
I comes under the animal welfare act, if you want to look it up, but in short-
S4 of the 2006 Animal Welfare Act says that a fish is the owners responsibility and they have a duty of care, legally you must keep the pet in suitable conditions (e.g. shelter, food, water) and you are responsible for the fishes welfare. The owner has a duty to see that the fish does not suffer unnecessarily; it must be protected from disease, pain and injury.
So feeding a live fish to another fish that is willing to eat non-live fish could be seen as the owner causing the feeder fish unnessarary suffering and even the predetor fish if it was injured while trying to eat the other fish. The law doesn't specificly say you can't but it is in the grey area- the courts could interpret the law either way.
Sorry I can't actually answer your question but I figured you would like to know how legal/illegal it is... and i have a Law exam coming up and this was good practise ;p