'Diver' (i) was the British overall system to provide defence against the German V-1 flying bomb (June 1944/May 1945).

As part of this undertaking, anti-aircraft guns were redeployed in several phases: first in mid-June 1944 from positions on the North Downs in Kent and Surrey to the southern coast of England, then a cordon closing the estuary of the Thames river to attacks from the east, in September 1944 a new defence line along the coast of East Anglia, and finally in December a further line along the coast of Lincolnshire and Yorkshire. These successive deployments were prompted by the ever-changing approach tracks of the missiles, which were in turn influenced by the Allies' advance through Europe and then the German adoption of aircraft as launch platforms.

Anti-aircraft gunners found that small, fast-moving targets such as the V-1 were difficult to track and to hit. Initially an average of 2,500 shells had to be fired to bring down just one V-1. The average level-flight altitude of the V-1 was between 1,970 and 3,280 ft (600 and 1000 m), and this was a narrow band between the optimum engagement altitudes for light and medium anti-aircraft weapons. These low altitudes defeated the rate of traverse of the standard British 3.7-in (94-mm) mobile AA gun, and static gun installations with faster traverse rates had therefore to be built at great cost.

Balloon barrages were also deployed against the missiles, but the leading edges of the V-1’s wing were equipped with balloon cable cutters and fewer than 300 flying bombs are known to have been crashed as a result of hitting cables.

Fighter defences were also mobilised as part of 'Diver' (i). Most fighter aircraft were too slow to catch a V-1 at its cruising altitude unless they started the engagement with the advantage of higher altitude, allowing the fighter to dive on its target and in the process gain speed. Even when intercepted, the V-1 was difficult to bring down. Machine gun bullets had little effect on the sheet steel structure, and 20-mm cannon shells had a shorter range, which meant that the resulting detonation of the missile’s warhead could destroy the intercepting fighter as well as the target. The V-1 was also nearly immune to conventional air combat techniques because of its design, which eliminated the primary 'one-shot stop' points of pilot, life support and a complex engine: the V-1 had neither pilot nor oxygen system, and the V-1’s Argus pulse-jet motor was extraordinarily simple in concept and construction, and could thus be shot full of holes yet still continue to provide sufficient thrust for sustained flight.

The only vulnerable point was the valve array at the front of the motor, and the only one-shot stop points on the V-1 were the bomb detonators and the line from the fuel tank, three very small targets buried inside the fuselage. An explosive shell from a fighter’s cannon or anti-aircraft artillery was the most effective weapon, if it could be detonated in a direct hit on the warhead itself.

When the V-1 attacks on the UK began in mid-June of 1944, the RAF had fewer than 30 Hawker Tempest single-engined interceptors in No. 150 Wing with which to provide an effective air defence against this primitive yet effective cruise missile. Few other aircraft had the low-altitude performance for a realistic chance at air interception.

Initial attempts to intercept V-1 missiles often failed, but the British interdiction techniques developed rapidly. These included the hair-raising but effective method of using the airflow over an interceptor’s wing to raise one wing of the Doodlebug, by slipping the tip of the interceptor’s wing under the wing of the V-1 and raising it to within 6 in (0.15 m) of the lower surface. When this was done properly, the airflow would tip the V-1’s wing up, overburdening the ability of the missile’s gyroscope-based control system to remedy the bank angle and sending the V-1 into an uncontrolled dive.

The Tempest wing was built up to more than 100 aircraft by September. Other aircraft used in the task included the North American P-51 Mustang and the Rolls-Royce Griffon-engined Supermarine Spitfire Mk XIV single-engined fighters, which were carefully polished and tuned to boost their speeds, and during the short summer nights the Tempest fighters shared the defensive duty with de Havilland Mosquito twin-engined aircraft. Republic P-47M Thunderbolt single-engined heavy fighters, modified by the removal of half their fuel tanks, half of their standard armament of eight 0.5-in (12.7-mm) machine guns, all external fittings and all their protective armour, were also pressed into service against the V-1.

There was no need for radar, as the V-1 was distinctively visible by day and at night was revealed by the long plume of incandescent gases streaming from the pulse-jet motor’s exhaust. Moreover, the sound of the engine could be heard from 10 miles (16 km) away. Daylight V-1 chases were chaotic and often unsuccessful until a special defence zone between London and the coast was declared: in this only the fastest fighters were permitted to operate, free from all fear of being engaged by the anti-aircraft guns.

Between June and the middle of August 1944, the handful of Tempest fighters shot down 638 flying bombs. One Tempest pilot, Squadron Leader Joseph Berry of No. 501 Squadron, downed 59 V-1 missiles, and Wing Commander Roland Beamont destroyed 31. In descending order of the 'kills' they scored, the other major contributors were the Mosquito (428), Spitfire Mk XIV (303) and Mustang (232), and all other types combined 158. The still-experimental Gloster Meteor twin-turbojet fighter, which was rushed half-ready into service to fight the V-1, had ample speed but suffered from jamming of its four 20-mm cannon, and therefore accounted for only 13.

By mid-August 1944 the threat had been all but overcome not by the defending aircraft but by the arrival of two enormously effective electronic aids for anti-aircraft guns, both developed in the USA by the Massachusetts Institute of Technology’s Radiation Laboratory: these were radar-based automatic gun-laying using, among other equipments, the SCR-548 radar, and most importantly of all the proximity fuse. Both of these had been requested by Lieutenant General Sir Frederick Pile’s Anti-Aircraft Command and arrived in numbers, starting in June 1944, just as the guns reached their free-firing positions on the coast: 17% of all flying bombs entering the coastal 'gun belt' were destroyed by guns in the first week on the coast, and the kill rate rose steadily week by week to reach 60% by 23 August and 74% in the last week of the month, when on one day 82% of all targets available to the guns fell. The kill rate thus improved from one V-1 for every 2,500 shells fired to one for 100 shells fired.