That is a good question. Since the killing of general livestock is legal I will guess that animal sacrifice is also legal unless you are using an animal under the protected species list.
I have lived in a Pagan Community in Britain for many years, and have never come across it.
No Wiccans, Druids or Shamans I have known would advocate it. Many of them are vegetarians and vegans, and most of us that do eat meat find this sacrifice sufficient.
We may be Neo-Pagan, but the sacrifice of poor animals is something we do not do.
I've never thought about it in legality terms? I may have to look that up, but the word 'sacrifice' in any way has a negative connotation, so I don't think it's a thing that would be legal here?