The UK Ministry of Defense revealed some details about its nuclear warhead dismantlement program in its response to a Freedom of Information Act request filed by journalist Rob Edwards (pdf copy of the MoD letter).
The warhead dismantlement program is part of the UK commitment to reduce its overall nuclear stockpile to no more than 180 warheads. According to the MoD letter, "by May 2010, the number of warheads in the stockpile had been reduced to fewer than 225" and the program is expected to be completed in the mid 2020s. The letter also stated that
Since 2002, the Atomic Weapons Establishment (AWE) has been running a Stockpile Reduction Programme to disassemble Trident warheads to reduce stockpile numbers as declared in the Strategic Defence Review 1998, the 2006 White Paper on the Future of the United Kingdom's Nuclear Deterrent and the SDSR. The warheads that have been identified as no longer required for service but are yet to be disassembled are stored at the Royal Naval Armaments Depot Coulport or as work in progress at AWE Burghfield. All warhead disassembly work is undertaken at AWE Burghfield.
The main components from warheads disassembled as part of the stockpile reduction programme have been processed in various ways according to their composition and in such a way that prevents the warhead from being reassembled. A number of warheads identified in the programme for reduction have been modified to render them unusable whilst others identified as no longer being required for service are currently stored and have not yet been disabled or modified. This is in line with the overall target date to achieve the declared reduction by the mid 2020s.
Independent monitoring of nuclear convoys performed by Nuclear Information Service suggests that each year about three warheads are transferred to AWE for disassembly.
Earlier, the UK government stated that "the material from dismantled warheads is returned to the MOD nuclear material stockpile. It is not government policy to place this material under international safeguards." It is not clear if this material is considered excess to military requirements as the last UK declarations of excess material, made in 2006 (HEU) and 1998 (Pu) refer to the audits conducted before the start of the current Stockpile Reduction Programme in 2002.