I'm not in a position to comment on BIAZA's (or DEFRA or BALPPA for that matter) actions so cannot answer that question. Perhaps you could email their office? Perhaps we could, at the same time ask BALPPA why they haven't provided updated info for their members? Perhaps they are not as proactive at seeking clarity as BIAZA? It's just a case of making details available as they come to light.
Of course you, and the rest of the zoo community, must do what is right. For the record BIAZA's advice is, and was always, to keep details of guests - even though (as has only just been confirmed) it is not a legal obligation for zoos.