The relief for a guard dog is available as a deduction from profits. A guard dog is considered as “plant and machinery” for the purposes of taxation so you can claim relief on a dog’s “running costs” such as food stating it's a guard dog. As you would expect, any bone of contention from HMRC will be whether the expenditure was WHOLLY & EXCLUSIVELY incurred for the purpose of the business.

If the dog is also a family pet but is used sometime as a guard dog, you should still be able to claim a share of the costs.

These costs are claimed under CA21220 – Plant and machinery

See this link to the HMRC Website:
http://www.hmrc.gov.uk/manuals/camanual/ca21220.htm