To be honest your best speaking to a specialist accountant specializing in agriculture -
Snipets from advice I got and paid for;
If its conceivable you could make a profit, register for vat and treat it as a business, the vat claim back alone on is 20% of your expenses, so even a small flock of 30-50 ewes could get £400 or so back in Vat.
Again, fencing your field? new gate? claim your 20% back.
If you make a trading loss - IE sell your lambs and get bad price at market, costs higher and a few ewes die, bad winter etc... you can offset the loss you make against tax in future years.... meaning if you make a profit next year, you may not need to pay as much or any tax on it, as they take into account your loss the previous year (s)/.
Also their is a size issue - if your keeping sheep and a few chickens in numbers that are just too small...... its basically a hobby - ie half a dozen chooks and 2-3 sheep - you simply could not make any money from that after your expenses....
It gets more complex and messy but an accountant should be able to tell you pretty quick where you stand;
In my case it was - Register as a business as I had plans in place to grow my flock to 35+ breeding ewes, and have the land in place, to support revenues of 5-7k based on similar farm business.... Which meant that although costs are likely to trail close behind the revenue, their is a strong chance of profit in some years.... and more so if you can claim back the vat - as a 15ac site is enough to mean expenditure where the vat can run into the £100's.