Britian has been fighting to protect its borders or someone else's for centuries. There have been civil wars and colonial wars.
Unfortunately, one of the problems with researching military records, particularly in the United Kingdom, is the widespread physical locations of the material. The National Archives (UK) at Kew, West London is as close as you will get to a 'one stop shop' and an excellent place to start. Their informative website at www.nationalarchives.gov.uk has details of the services on offer, as well as detailed research guides and a growing collection of online records. While some research can be done online, most requires a personal visit to the National Archives (UK), or, if this is not possible, you can engage a freelance researcher.
Army service records for World War One are available at the National Archives, and many are available via Ancestry.com (available for use at the State Library of SA). Sadly, about 60% of the records were destroyed during bombings in 1940, and others suffered badly from fire and water damage. You must therefore be prepared for the possibility that the file you are interested in may not have survived.
After the restoration of the monarchy in 1660, military records became more plentiful. The main War Office records contain a great deal of information. Records for officers and other ranks up to the start of the 1st World War are kept at the National Archives (UK) at Kew in Richmond, London.
Records since then are in the custody of the Ministry of Defence. For further information, contact the Army Historical Branch, Old War Office Building, London SW1A 2EU.
Information concerning naval personnel can be obtained from the Naval Historical Branch, The Admiralty, London. The Admiralty Records are also in the National Archives (UK) at Kew.
Records of the Royal Marines are also in the National Archives (UK), but many of the early documents are lost.
The following links will take you to indexes available via Find My Past. You will be able to search for information and see search results, however to see original records or transcripts you will need to pay as you go or use the subscription database Find My Past.
The State Library of South Australia has a subscription to Find My Past that may be used within the Library.
While you are in the Library don't forget to check out our databases to see what military information they hold: