Asked an acc this while droping off last of my self ass this morning - In short IF you sell anything to do with livestock, If you just keep pet farm animals, but take SFP/BFP or eat the animals - Register....
Even if you consider it a "hobby"... the problem is its capital intensive and thefore HMRC will want their cut.
Long term its better to be so anyway to get the vat back, especially if your marginally profitable.