It is now possible to search the U.K. National Archives'(formerly Public Record Office's) catalogue online, on the web. Some records have been digitized and access is available to them on their site. Their web site has all kinds of useful information about planning your research visit to this facility. It is the national repository for the U.K.'s government materials.
Formerly, researchers had to go to their facility in outer London to identify useful materials and study them. Or they could use the microfiche set called The Kew Lists described here, to identify which materials would be useful prior to going abroad.
Great Britain. Public Record Office. Kew Lists. [London], HMSO Books, 1988, c1986, and c1991. Microfiche CD 1043 .K48 and Supp. Hollander Make Central 2 West
The Kew Lists are the finding aids used to identify the call numbers for materials in the National Archives (formerly the Public Record Office), which is the national repository for Britain's government materials. Use the Public Record Office's Current Guide (Main CD 1043 .A562) or the set of the Current Guide located at the beginning of the whole fiche set in Microforms, first. Go to Part 3: Index, to look up your government related topic. This Index guides you to the appropriate numbers in Part 2: Class Descriptions and/or Part 1: Administrative Histories. Having identified your class description numbers, you then find these numbers in the larger microfiche set of the Kew Lists in Microforms in the M.S.U. Libraries. Here you will find detailed information and the appropriate call numbers to take with you to the Public Record Office, where you will be able to conduct your research in Britain's official governmental archive. The M.S.U. Libraries only owns the index to the materials, not the materials themselves.