This needs splitting into three areas
1.
The "breeds at risk register" contains a list of the breeds that might be considered eg British Saddleback Pig, West Corwall short eared goat, welsh valley tree-climbing cow.
so the actual register is only the names of breeds, not any details of breeders holders etc.
2. You can submit your name onto a list (
http://www.defra.gov.uk/animal-diseases/controls/culling-exemptions/), but in all honesty I cannot see how anyone would use this and I expect this to be junked as out of date partial data, and the site says in any case " During an exotic disease outbreak, keepers with premises within control zones or on suspect/infected or contact premises will also have the opportunity to inform the AHVLA if they have breeds at risk when AHVLA officials visit their premises."
3.
Which breeds go on the list. Defra use sources such as the BPA and the British Lop club to obtain details and evidence that breeds are sufficiently rare to go on the list.
If/when an outbreak occurs, then any farm that comes under potential cull can use the fact that they have livestock from breeeds that on the register as a defence to plead that their rare bred stock should not be culled. They would need to show that their stock was indeed these breeds, so registration on either BPA or equiv for other species or relevant breed society (eg British lop) would be needed, and probably checked.
However EVEN in these circumstances, their stock may still be culled - as the defra websire says "A principle of exotic disease control legislation is that certain categories of animals/birds (see below) may be considered for sparing from culling, provided that disease control is not compromised. However, such exemptions for both animals/birds are not guaranteed and will be considered on a case-by-case basis following a veterinary risk assessment of the premises and consideration of wider issues and impacts."
So
a) no-one holds a list of "holdings" that are exempt - each holding is looked at only as potential cull of stock is looked at.
b) You can go on a "pre-list", but I really canp;t see anyone cbothering to use out of date partial info
b) As far as I'm aware from defra there are no minimum numbers (I believe the RBST number is a guide - BB have you got the link to where this is quoted?) - so someone keeping one rare breed pig of a rare bloodline and say able to show that they are protecting from potential infection (eg housing aawy from boundaries, foot dips etc.) might get spared, whilst a large farm may get culled anyway as they can't put any defence measures in place and are right next to a mega farm which could get infected via them.
c) The BPA registers etc. would not be used en mass, just as a confirmation of the stock being rare breed.