The taxonomic subcommittee of the BOU has recommended that Caspian Gull (Larus cachinnans) and American Herring Gull (Larus smithsonianus) would best be treated as separate species, rather than as subspecies of Herring Gull (Larus argentatus) as previously. The details of the recommendation can be read in full here.
Although this recommendation has not yet been accepted formally by the BOU Records Committee, this is considered to be only a matter of time and thus, BUBO Listing now allows the addition of these two species onto lists using the BOU or "Official British & Irish" authorities. [The two species were already split by the UK400 Club and Birdwatch authorities.]
This decision will be widely welcomed by British birders ("about time too" many will say!) and will result in a number of "armchair ticks". In recent years, Caspian Gull has proved to be a scarce but regular visitor, especially in eastern and central England; it remains scarcer than Yellow-legged Gull and, moreover, shows a less distinct late summer peak in the number of records. American Herring Gull is much rarer, with most records in western counties and identification of older birds still problematic.
[Note that the other recommendations made by the taxonomic subcommittee (i.e. changes to some latin names and alterations to the species order of some groups) will not be incorporated by BUBO Listing until they are formally accepted by the BOURC.]