The difference between hobby and business is not so clearly defined and can be different in every circumstance. Here is a little for instance
http://www.hmrc.gov.uk/manuals/bimmanual/BIM75710.htm What you have to think about, if you read the page and follow the explanatory click links I think you will see what I am getting at. If your breeding is a hobby, presumably it is from your pets and you would never hope to make much of a profit, but the profit you do make is taxable and there is no way round it.
When you start to add all the other things into your costs which are not related to actually producing a litter then you are looking at it more from a business point of view and then it is debatable whether you are producing puppies as a business or a hobby.
I have only produced two litters in recent years, and haven't had any for 20 years previous to that but I can be sure I will be asked about it because I also have a business which is totally separate from the dogs. I would not be claiming my puppies as a business but as a hobby and that limits my expenses to the actual whelping, vets bills and feeding of the litter and mum. If I were breeding 3 times a year for instance I would expect to put my showing, grooming, purchase of animals, stud fee, hip scoring and cost for the actual whelping and anything else I could think of against my profit from a litter and in the event that I made a loss I would carry it forward to the next year to set against my income for the following year.
I think the whole thing needs looking at again to be fairer to people who do only produce the odd litter. The IR seems set on tapping every reservoir of income it can think of.
Apologies to Daisy, I was getting my NI mixed up with my SE there is no allowance other than your personal allowance.