Britain has ordered 500 U.S. Hellfire II AGM-114-N4/P4 missiles. This is the one with a thermobaric (fuel-air explosive) warhead that is optimized for destroying structures. The Hellfire II weighs 48 kg (106 pounds), carries a 9 kg (20 pound) warhead, and has a range of 8,000 meters. In addition to UAVs, the missile is most commonly used by the AH-64 helicopter gunship. An AH-64 can carry up to sixteen Hellfires at once. Hellfire launchers are also available for AH-1W, AH–1Z attack helicopters, MH-60R, and MH-60S naval helicopters, OH-58D/Fs, and Harvest HAWK equipped KC-130J gunships. The heaviest user in the last five years has been UAVs, like the MQ-1 Predator, MQ-1C Gray Eagle, and MQ-9 Reaper. Britain uses AH-64a and MQ-9s.
The missile is popular for use in urban areas because the small warhead (with only about a kilogram/2.2 pounds of explosives) reduces civilian casualties. The missile is accurate enough to be sent through a window (OK, you have to be really good, and lucky, to do this) because of its laser guidance. Hellfire is the most frequently used missile during the war on terror.
Britain mainly uses its own version of Hellfire, Brimstone. The latest version of Brimstone uses its own radar to keep track of the target. The British 55 kg (110 pound) Brimstone was originally developed as an upgraded version of the American Hellfire. Brimstone ended up as a Hellfire in general shape only. Weighing the same as the Hellfire, Brimstone was designed to be fired by fast movers (fighter-bombers), not just (as with Hellfire) from helicopters and UAVs. Aircraft can carry more of these lightweight missiles. These are perfect for small targets, including vehicles that need to be hit without causing injuries to nearby civilians or friendly troops. This is what made Brimstone so popular in Libya.
A major factor in the success of Brimstone is its superior guidance system. Five years ago Britain added a dual-mode (radar and laser) seeker to Brimstone. Originally, Brimstone was to be just an American Hellfire with a British seeker (a miniature, millimeter wave radar) and configured to be launched from jets. Brimstone did that but never got a chance to show how effective it was until Afghanistan and Libya. The performance of Brimstone was particularly impressive in Libya because the missile was used so frequently. That got the Americans and French interested in using it as a highly effective anti-vehicle weapon for their fast-movers (jet fighter-bombers).