United Kingdom nuclear weapons
United Kingdom nuclear weapons evolved from close cooperation with the United States in the Second World War Manhattan Project, but Britain detonated its first fission device in October 1952, and has maintained an independent capability. At the present time, all of its nuclear weapons are in the form of UK-built warhead for submarine-launched ballistic missiles; it has retired all tactical nuclear weapons. Britain is a declaratory power under the Treaty on the Non-Proliferation of Nuclear Weapons.
The special relationship
The US-UK relationship in nuclear weapons is unique, but has varied in intensity both in development and in joint nuclear operations.
WWII and Aftermath
British scientists both were major contributors to the original U.S. bomb development; Klaus Fuchs, a very competent physicist, was also a Soviet spy. U.S. legislation, the McMahon Act, had cut off cooperation in 1946. 
Going it alone, the U.K. Atomic Energy Research Establishment (AERE) was set up, in January 1946, at Harwell, and soon work designing plutonium production. In parallel, weapons-specific work moved to the Atomic Weapons Establishment in Aldermaston. The first British fission device, Operation Hurricane, was detonated aboard an old warship, at Monte Bello off Australia's north coast, in 1952. The Soviet Union had detonated its first nuclear weapon in 1949, and the U.S. saw it as the primary opponent, responding with the IVY MIKE thermonuclear test four weeks after Hurricane.
Lord Cherwell told Winston Churchill, in 1953, that he thought Britain knew how to make a hydrogen bomb, etven though it was still working to build a stockpile of atomic bombs. Sir Norman Brook, Cabinet Secretary, and chair of the Home Defence Committee realized the thermonuclear tests, with a Soviet on in 1953, revolutionized defense planning. With the Chiefs of Staff, a report was issued called "Hydrogen bomb research and production in the United Kingdom", submitted to the Defence Policy Committee in June 1954, and the decision made to start a British thermonuclear program. 
Britain independently developed fusion techniques, beginning with the tamper boosting method developed by Keith Roberts. Independent British ideas made cooperation, when the U.S. was in intense competition with the Soviets, more attractive. 
Under the Harold MacMillan government, substantial exchanges began again in the late 1950s. President Dwight D. Eisenhower had supported cooperation, and gained additional authorization under the Atomic Energy Act of 1954, which allowed exchanges on external characteristics and effects of nuclear weapons, but not fabrication and design.  The British independently tested their first thermonuclear device in May and June of 1957, which the U.S. found interesting but not compelling. Things changed, however, with the Soviet launch of the Sputnik I satellite in October, and the beginning of the "space race" and an implied missile race. Assisted by the Secretary General of NATO, Henri Spaak, the sides issued a "Declaration of Common Purpose" on October 25.
Within the context of the declaration were both technology and operations. It allowed, for example, for the U.S. to station PGM-17 Thor intermediate range ballistic missiles in Britain, with the warheads under joint U.S.-U.K. dual key arrangement.  B-47 Stratojet medium bombers were also based in the U.K.
There was truly independent development by both countries, but, at various times, the British uniquely had access to U.S. bomb designs and research, and the British consulted on improving U.S. efforts.
British strategic weapons, in the 1950s, were carried aboard RAF heavy bombers. Both the U.S. and U.K. became increasingly concerned about the ability of conventional bombers to penetrate the Soviet Union, and the UK first focused on building its own intermediate-range ballistic missile program, Blue Streak. This was cancelled in 1960, however, because its land-based silos were considered too vulnerable.
The U.S. had been working on a long-range, bomber-launched air-to-surface missile, Skybolt, and Britain joined the program in 1961. In 1962, the Kennedy Administration, at the urging of Secretary of Defense Robert S. McNamara, who considered the delays and cost overruns unacceptable.
Cancelling Skybolt left Britain in a much more difficult situation. It had had a single means of delivery, while the U.S. was building its Triad of submarine-launched ballistic missiles, new bomber doctrine for low-altitude penetration, and much more secure intercontinental ballistic missiles.
The solution came when MacMillan and Kennedy signed the Nassau agreement, which made U.S. SLBMs available to be carried on British submarines, armed with British warheads. Signed in 1963, the policy continues today, with Vanguard-class ballistic missile submarines constructed and operated by the Royal Navy carry British warheads aboard purchased U.S. UGM-133 Trident D5 missiles.
While the UK had a strategic bomber force, it was being phased out by the 1980s. Nevertheless, in what had been the longest and most complex, multiply air-refueled bomber mission to date, a Vulcan aircraft bombed Argentinean-held airfields, on Falkland Island, during the 1982 Falklands War.
On 4 December 2006, the U.K. government reaffirmed its intention to retain nuclear weapons as the ultimate guarantee of security.
- The History of the UK’s Nuclear Weapons Programme, Ministry of Defence, Foreign and Commonwealth Office
- Donald McIntyre (2006), The Development of Britain’s Megaton Warheads, University of Chester, MA Dissertation, pp. 5-6
- McIntyre, pp. 20-27
- Lorna Arnold, Katherine Pyne, Britain and the H-bomb, Palgrave MacMillan, pp. 195
- Arnold & Pyne, pp. 199-200
- "Government announces intention to maintain the UK's Nuclear Deterrent", Defence News, 4 Dec 2006