I've been quite fortunate with WW1 records, most of my relatives records are still available. There are only about 1/3 or them left as the archives were bombed in the war. I think I've mentioned on here before, I've found records for relatives in 2/5 Staffs, the KSLI and the RN. If you haven't had a look I can really recommend searching for your ancestors - you need a full name, DOB, names of other relatives also help because WW1 army attestation forms have next of kin on them. The navy information that I have is much more sparse, just a page of a book listing the ships that my relative served on. I've attached some examples so you can see the sort of things that are available.
WW2 records haven't been generally released yet, presumably because there are veterans who are still alive. You can get request them but the process is more involved - you fill in some forms and you also need to get permission from the individual or the next of kin, see here :
https://www.gov.uk/get-copy-military-service-records. I managed to get my Grandfathers records this way. He was on the home front initially driving all over the country, he went over to France in late June 44, ended up on the X list a month later. He was assigned to another unit when he was fit again and then made his way up through Belgium, Holland and ended up in Nothern Germany. It looked like he was attached to the 5th Armoured Dvision, the famous desert rats. I have one of their patches somewhere and always thought that he was in the desert before I got his records. Amazingly we found that he was at RAF Uteresen at the same time as my partners Aunt in late 1945.
The above post probably contains sarcasm or some other form of attempted wit, please don't take it to heart.