World War I can be a tough one to research, unfortunately, I have discovered the hard way. It was so long ago that it is hard for personal documents—without great care having been taken for their preservation—to have lasted this long. Access to survivors, if there are any left, would be minimal and likely widely impractical. Interviews may survive, but may have nothing to do with the person or persons you are looking for. Of course, there was also the fact that the War Office was bombed during the second war. Many of the service records kept from the previous war were destroyed, and most of the information left is pulled from pension requests, so that can be a problem if the person you are looking for died either during the war or before they were able to request a pension. Unless they were Household Cavalry, then you are likely fine, as long as you don’t mind making a written request to view them and then going to the Cavalry Museum in Berkshire to see it. Some of the Guard Regimens also had records destroyed, but you have to contact the individual regime in question in order to see if your record was one of them. These records form two indexes, the “burnt documents” and “unburnt documents” such as the pension records.
The National Archives has most of what’s left on microfilm, accessible at their location in Surrey. If you cannot get there, never fear. They granted access rights to Ancestry.co.uk and findmypast.co.uk, and with a subscription, you can see the same documents without any traveling. Plus you can search for the people you are looking for first, for free. Saves you a trip if your family member was part of the burnt records.
However, even if the record you need was destroyed in the bombing, you have some other choices, as long as you think logically. The London Gazette has had a longstanding tradition of publishing every military appointment as well as awards, and most were eligible for some type of acknowledgement. You can visit the Imperial War Museum in London and look for it there, or search the archive online through the Gazette site. You can try your hand this way to locate someone and verify that they served. Also, the Medal Rolls themselves survived, and can provide a lot of information (although it does depend on how thorough the person who filled out the card felt like being at the time). Those have been digitized and scanned, so they are searchable and viewable on Ancestry.co.uk. These cards are considered a much more accurate representation of men and women who served, again the logic being that most people qualified for something during their time in service.
If you are finding yourself stuck for information during this time period, you are not alone. I would consider purchasing a reference guide to help or hiring a genealogy expert who has experience finding records of people who served in World War I. It can be difficult, but if you think outside the box enough, it is not impossible. Keep digging, and good luck.