The enzyme reference was on the feeding of milk products for calcium when there are lots and lots of other sources which are healthier for birds (which are not mammals so they lack the necessary enzymes to break down milk or its products).
As to following avian vets recommendations on diet, with all due respect, I don't know what the avian vets study in England but here in the States, they don't study avian nutrition at all (I am not an avian vet but I have the text books) and I doubt it's different in England because, if they did, they would need another 5 years of schooling. Avian nutrition is a field that is in its infancy and every species has a different nutrition ecology and dietary needs (most of them unknown) so the only way to learn is through constant research (studies and field biologists reports) and keeping many different species for a loooooong period of time (and that doesn't include seeing them in an office but living with them day in and day out) so, in reality and contrary to what most people believe, your average avian vet doesn't really know much more than the basics and, in most cases, much less than aviculturists (same goes for behavioral problems). There might be a very few that have put the time and effort into learning and who have kept parrots for a long time but I've been keeping them since 1992 (all rescues) and although I've dealt with, at least, 15 to 20 different AVs, none of them knew diddly squat about it and just repeated the 'feed them pellets' mantra - even for canaries and finches which are natural seed-eaters and which I've been keeping VERY successfully for 45 years.
If I sounded as if I was telling you not to follow your vet's advice, I apologize, that was not my intention. I am just giving you my informed opinion which I am not expecting you to follow just on my word, of course. I wanted to give you a different perspective so you would do your own research and see if you agree or not with your vet because, for one thing, pellets (even TOPs which are the best in terms of pellets as they have no soy or added man-made vitamins) are not the healthiest option, especially for tiels and other small species. They are too dry and end up causing kidney problems because parrots are not hard-wired for drinking a lot of water as they are never meant to eat anything with low water content (plant material, their natural diet, is between 80 and 95% water -check the label for the moisture content on the pellets and you'll see they are only about 4%) and are, instead, meant to derive most of their hydration needs from their food. Now, some birds dunk everything in water (I have a LSC2 that does that even with birdy bread which is VERY moist) so if yours are doing this, then I would assume it should be fine.
I also feed organic -not everything but definitely the items that need to be (the USDA puts out a list every year with the 'dirtiest' and the 'cleanest' produce) and whatever other item that might not be but is easily available and relatively unexpensive. This is not only because feeding everything organic is hugely expensive (I rescue and have a lot of birds) but because there are items that cannot be found organically grown (my parrots eat a HUGE variety of fresh food).





