The Penney Report, outlining the features of an atomic bomb based on the U.S. Fat Man pattern, and the tasks required to develop one for Britain, was declassified and made available to the the public under the Public Records Act (now amended by the Freedom of Information Act which came into force in January 2005).
The UK Public Record Office File AVIA 65/1163, "Implosion" (covering the years 1947-1953) has been withdrawn from public access since 2002 and this will not be reconsidered until 2014. The actual legal status of this file remains as a public record. Its access condition has been changed to "Retained by Department under Section 3.4" (of the PRA) which means that the file has been returned to the custody of the originating department (Ministry of Supply) or its successor. This limitation of access does not constitute reimposition of a secret security marking, and no attempt appears to have been made by the UK government to contact people who had previously obtained copies of this file.
It should be observed that Penney's description and discussion of development are no more revealing than descriptions of the United States' first implosion bomb that have been publicly available for many years, and in fact are less precise than other descriptions that are now available, and are in any case are the oldest material in the file, written before any actual bomb development work had been undertaken in the UK.
Penney's description is by all accounts far less technical and detailed than "Tuck's Bible", detailed notes written around this same time by James Tuck, another member of the British Mission to Los Alamos who, unlike Penney,was deeply involved in the actual design and devleopment of the implosion bomb. Tuck's Bible has never been made public.
William G. Penney, the British "Oppenheimer", spent most of 1944 and 1945 as part of the British Mission at Los Alamos. This was an elite team of British scientists, and emigres to Britain, who contributed to the development, testing and use of the atomic bombs during the Second World War. Sent as a specialist on ocean waves, his gifts were readily apparent at Los Alamos and he soon was made one of the five members of the Los Alamos "brain trust" that made key decisions in the direction of the program, putting him in the company of Robert Oppenheimer, John Von Neumann, William "Deke" Parsons and Norman Ramsey. On 27 April 1945 Penney became one of only two representatives from Los Alamos (and the only Briton) to be part of the ten man Target Committee that drew up the list of targets for the atomic bombing of Japan. Penney travelled to Tinian Island in the Pacific to be on hand for planning and briefing the atomic bombing missions. Penney actually witnessed the bombing of Nagasaki, flying in an observation plane accompanying the attack. Afterward he conducted damage surveys there on the ground.
Penney returned to Imperial College immediately after the war, but accepted an appointment to head the Armament Research Department (ARD) on 1 January 1946. On 8 January 1947 the secret GEN.163 Cabinet committee of six Ministers (headed by PM Attlee) decided to proceed with development and acquisition of atomic weapons. Penney did not receive word of this decision until May 1947 when he was finally asked by Lord Portal to lead Britain's own nuclear weapon program. The decision was not disclosed publicly in any respect until 12 May 1948, when an oblique reference was made to atomic weapon development in parliamentary discussions.
In June 1947 Penney began assembling a team to work on the bomb. One of his first steps was to prepare a document describing the features of the U.S. implosion bomb, breaking down the development tasks required to replicate it, and identifying outstanding questions that required further research. This report was completed on 1 July, was entitled Plutonium Weapon - General Description (UK Public Record Office File AVIA 65/1163, "Implosion") and gave the British atomic weapons program a preliminary design description roughly equivalent in terms of detail to the description provided the Soviets by Klaus Fuchs.
A much more detailed technical discussion, known as "Tuck's Bible"was created by British Mission member James Tuck, who had been instrumental in the development of the explosive lenses used in the implosion bomb.