A British breed is one that was originally created on the island of Great Britain, which includes England, Scotland, Wales and the associated islands of Shetland, Orkney, Inner and Outer Hebrides, the Isle of Wight and other small islands, but not including the independent crown dependencies of the Isle of Man or the Channel Islands. These do not form part of the island of Great Britain. However as they have a close association and close cultural and geographical relationship with Great Britain breeds developed in these dependencies can be classed as British. Northern Ireland forms part of the United Kingdom and is not part of the Island of Great Britain. 

Imported breeds originally developed outside of Great Britain that are extinct or considerably rare in their country of origin cannot be classed as British as they were not developed in Great Britain. It would be unfair to take credit for a breed that was developed by another country. Imported breeds that have a long period of development within Great Britain, but have not been crossed with another breed to the extent that the crossing affects all stock cannot be classed as native they are truly British versions of an established imported breed. An imported breed must first be crossed then selected to be unique and then renamed to be classed as a unique native breed. 

The origin of a breed is determined by studying historical documents and publications referring to the breed. In cases where breeds have existed on the island of Great Britain in time immemorial, perhaps for millennia, historical information is not available and may be unreliable. In such cases these breeds can be classed as native as it is likely they have evolved and probably been crossed sufficiently to become a unique breed of Great Britain.