The National Archives, or Public Record Office as it was known until very recently, is the leading repository of our national history.

Covering a large site in south west London its holdings are far too numerous to detail here, but perhaps the best known and most utilised from the point of view of family history, are its collections of material relating to the armed forces, and in particular those covering service in WW1.

Alongside its military archive should stand  the archives of the Merchant Marine, non-conformist church registers, wills pre; 1858, immigration and emigration, the Coastguard and Customs services, the Metropolitan Police, Railway company staff records, criminals and criminal proceedings, and the Prison Service.
This list is of necessity extremely perfunctory, and does not include a wealth of other material of value in any program of family history research.