Dear Mr. Dedeyan,
Thank you for posting your request on History Hub!
If you are referring to British citizens who joined international brigades to fight on the Republican side of the Spanish Civil War, please contact the Russian State Archive of Socio-Political History (RGASPI) as they have holdings relating to the international brigades. There also are records related to international brigades at The National Archives of the United Kingdom. Plus, there may be relevant records at one or more of Spain’s archives. We do not know if any of these institutions have service records of individual volunteers or disciplinary and courts-martial records for units.
We hope this information is helpful. Best of luck with your research!