There follows a list of archives holding useful substantial material (in English) on British volunteers and the Spanish Civil War. All archives listed are UK based, with the exception of the RGASPI archives in Moscow, which can be accessed online.

International Brigade Collection, Russian Centre for the Preservation and Study of Recent Historical Documents, Moscow.
Labour History Archive and Study Centre, People’s History Museum, Manchester.
Manchester History Archive, Tameside Local Studies and Archives Unit, Ashton-under-Lyne.
National Archives, Kew.
Spanish Civil War Collection, Imperial War Museum Sound Archive, London.
Spanish Civil War Collection, Working Class Movement Library, Salford.
The Spanish Collection (formerly the International Brigade Archive), Marx Memorial Library, London.
Swansea Miners’ Library, Swansea.
Trades Union Congress Archive, University of Warwick.