Yes, the law is againt using live vertibrates as food. However, it is not illegal to feed dead vertibrates. So killing the fish before giving them to the preditor would not be breaking the law.
It also means that I cannot feed live goats to my komodo dragon and he has to make do with bigmacs instead.
In a number of reptile houses in France it is common to feed the snakes live chickens/ rabbits for public 'entertainment'. These animals can survive for considerable lengths of time (hours to days depending on the snakes hunger at the time) in an entirely unnatural situation with no prospect of survival. I do not think that a situation like this is either humane or desirable in the UK.
I agree that it is far harder to justify banning the use of live fish on cruety grounds, although vertebrate/ non vertebrate life seems to be as good a place as any to draw the line.
Ed
edited for clarity