The Special Air Service is a corps of the British Army. During Hellsing's raid at the Baobhan Castle, the SAS 22nd Regiment ordered Hellsing to stand aside, apparently under orders to apprehend supposed IRA terrorists. It is possible that the vampire Incognito deliberately leaked information to the SAS about the possible terrorists both to delay Hellsing and to provide himself with troops as the entire SAS unit was later turned into canine-like FREAKs. Incognito later ordered the newly transformed SAS vampires to attack Cambridge and Trafalgar Square as a means to lure Hellsing to the Tower of London.
Hellsing forces arrived at the Tower of London though it was later revealed to be a trap to frame them as an enemy to England. Hellsing's special forces unit were quickly overwhelmed by the SAS vampires and were eventually wiped out. Alucard and Seras Victoria were able to destroy the remaining SAS vampires along with Incognito.