SAS

The Special Air Service (特殊空挺部隊 とくしゅくうていぶたい ,), shortened to S.A.S, is a special forces corps of the British Army.

They specializes in counter-terrorism, hostage rescue, ect. and will even interrupt another organisation to do so. While Hellsing's responsibility is to killing supernatural creatures, they seem to know about that too; and their leader is likely apart of the The Convention of Twelve.

However, the captain of the S.A.S specifically stated that it does not matter if a wanted terrorist is not a human anymore; that does not mean their guilt as terrorists can be erased.

Special Air Service
The Special Air Service traces its origins to 1941 and the Second World War. It was reformed as part of the Territorial Army in 1947, named the 21st Special Air Service Regiment (Artists Rifles). The 22nd Special Air Service Regiment, which is part of the regular army, gained fame and recognition worldwide after its televised rescue of all but two of the hostages held during the 1980 Iranian Embassy siege.

While it's allied with the Hellsing Organisation, it has a complicated relationship despite both being willing to trade soldiers with one another and the SAS is important to the Hellsing Organisation; many of whom were in SAS, was transferred into the Organisation itself had they need more soldiers to fight. However, episode 9 of the Hellsing anime, Sir Integra noted that they wouldn't get that much skilled soldiers from the SAS after the Valentine Brothers attack on the Hellsing Manor; meaning that the members of the SAS were likely aren't that skilled.

In the Manga & OVA
During the operation on the HMS Eagle, two SAS helicopters were destroyed (along with the SAS members inside) by a bullet from Rip Van Winkle's musket.

In the Anime
During Hellsing's raid at the Bobhan 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.

Equipments and Abilities
They're fully armed, even more so than Hellsing soldiers; supporting an NBC suit, which stands for "nuclear, biological, and chemical", to avoid any direct contact with radioactive or dangerous substances which can lead to death; which, it was made specially to do just that. They're colored dark to avoid detection from enemies, and wore a black vest along with a black combat boots. They also mostly uses an MP5 submachine-gun as their main weapon. Side weapons includes a Glock, and some uses an Enfield L85. As a ghoul, they appear to be more like a werewolf instead of an undead zombie-like creatures, and they also appear to be more fast and stronger than that of an average ghoul; making the Hellsing organisation have a hard time fighting them, and to be precise; they had almost completely wiped all of their soldiers, had Alucard or Seras not join the battle.