As you know Joy, we are also registered with DEFRA because of the poultry, but we are also registered as a smallholding because we had goats when we moved here.
I think the main advantages to me is feeling reasonably good about being notified if there are outbreaks of things like avian flu etc, but also because when you are trying to find out information on stuff animal related, or need help about things or even trying to buy equipment and feed etc cheaper, you get much further if you can say you are a smallholding, rather than just have a large garden!!!! lol