This is from the BBRC website:
4.1.1 Definition of a BBRC Rarity
The starting point for defining a national rarity is either 100 or fewer individuals in the most recent ten-year period, or ten or fewer individuals in at least seven of the last ten years. Any taxon that meets, or is close to, either of these criteria, will then be considered for inclusion as a BBRC rarity.
The Committee will also consider other factors such as population trends, distribution changes and identification issues.
4.1.2 Removal of a BBRC Rarity
All taxa currently assessed by BBRC will also be looked at, to check they remain rare. The starting point for removing a national rarity is either more than 100 individuals in the last ten years, or more than ten individuals in at least seven of the last ten years. Any taxon that meets, or is close to, either of these criteria, will then be considered for removal.
The Committee will also consider other factors such as population trends, distribution changes and identification issues.
Once a taxon has been dropped as a BBRC rarity it would not normally be reinstated, even if the criteria of Section 4.1.1 are met, until 10 years have passed.
https://www.bbrc.org.uk/about/constitution/4-bbrc-taxa