I'm not a fan of live feeding, and certainly not goldfish to tropicals, but:
It could be construed as illegal in the UK, though you would have to prove criminal intent beyond all reasonable doubt, which would be close to impossible (I used to have to try and prove criminal intent for a job).  There is no law saying it is illegal to feed fish to other fish.
The law only states that if you feed a vertebrate to another animal in such a way that it suffers unnecessary pain or stress it is illegal (though the act this is in spends half of its time talking about knacker's yards as it is so old).  The new act is almost certainly not going to ban live foods due to the difficulty in getting some fish and reptiles onto dead foods (they need the weaning period).
And look at it this way, was the way that goldfish died any more painful and stressful than meat slaughtered for Kosha or Halaal?  It was over in like what, 8 seconds?  Take fish swallowed whole (such as many people give to Oscars and such).  Is the death of that fish, through suffocation in the stomach or slow digestion by stomach acids, really less stressful and painful?