You may be starting with existing information that will have pointed to your ancestor as having served in the army and if you are lucky enough, you will have enough detail to start a comprehensive search based on that information.
Begin your searches online by clicking on this link:
British Army Service Records 1760-1915.
These online records comprise of:
Militia service records 1806-1915 (WO96)
Chelsea Pensioners British Army service records 1760-1913 (WO97)
Royal Hospital, Chelsea: pensioners’ discharge documents 1760-1887 (WO121)
Royal Hospital, Chelsea: pensioners’ discharge documents, foreign regiments 1816-1817 (WO122)
War Office: Imperial Yeomanry, soldiers’ documents, South African War 1899-1902 (WO128)
Royal Hospital, Chelsea: documents of soldiers awarded deferred pensions 1838-1896 (WO131)
These records are made available through www.findmypast.co.uk in association with The National Archives. These databases comprise of the records of ordinary soldiers of ranks other than that of officer.
The army records can be quite comprehensive and often reveal physical appearance details as well as service achievements. The Militia records also state the profession of the individual as nearly all the men who served in the Militia were part-time army personnel and when not on army service would have held down regular jobs. Up to the 20th century, all males in England and Wales had to serve in the Militia.
Campaign Medals and Award Rolls
For campaign medals and award rolls, search online here:
UK, Military Campaign Medal And Awards Rolls, 1793-1949.
This is a database listing over 2.3 million officers who received medals and rewards for service in British Army campaigns and battles.
Included in the online database are medals awarded for British Campaigns in:
The Middle East
West and Central Africa
The medals in this collection were awarded for:
Campaigns or Battles
Good Conduct (or Long Service)
Please note: this collection doesn’t include the medals and award rolls of World War I or World War II.
The War Office compiled the original medal rolls and these are now at The National Archives in Kew, London and can be searched there if you know the regiment and the campaign.
The original records of individuals who served in the British Army are also kept at the Public Records Office at Kew in London, but original Regimental records may also be found at the Chelsea National Army Museum or in the archives of the named Regiments.
British Army soldiers after 1913
Military Records at Ancestry.co.uk
Military, armed forces and conflict – FindMyPast
Get a copy of military service records – gov.uk
British Army WW1 Service Records, 1914-1920 (Soldiers)