The scans of the first batch of WW1 army records are now available for browsing at your nearest LDS Family History Centre or at the Family History library in Salt Lake City. These records are also available at The National Archives in London where the originals are kept, as well they are indexed and available through Ancestry.co.uk.
The records date from the outbreak of WW1 in 1914 through to 1920. They comprise two sets of documents WO 363 which are the enlistment papers which were damaged in WW2 and are generally referred to as the “Burnt Documents” and WO 364 which are Pension Claims made by soldiers who suffered some disability due to service in WW1. It is worth bearing in mind that the Pension Claims include claims made by soldiers which weren’t accepted by the War Office, so if you have heard that your ancestor didn’t get a war pension this doesn’t mean he might not appear in this record set.
The Family Search website has a good wiki on WW1 Army Records available online at https://familysearch.org/learn/wiki/en/United_Kingdom,_World_War_I_Service_Records_(FamilySearch_Historical_Records)
The National Archives, London also has an online guide at http://www.nationalarchives.gov.uk/records/looking-for-person/britisharmysoldierafter1913.htm