|Masters of the Hound, ca WWI|
There are two things I like to find in a family history research tool: (1) a large set of names to search on, ideally, a million or more, and (2) a price tag of absolute zero! You just can't beat free family history searches.
The Military Records Search at Forces Reunited does the job on both counts, though it just squeaks by on #2...you can search for free, and get preliminary results on members of the UK military, both present and past. But if you want the full record, you'll need to subscribe to the Forces Reunited site.
Still, a million-plus records makes for a pretty good, powerful search, and the free results are enough to let you know if there's more information in the dataset that makes it worthwhile to sign up but as numerous comments note, they try pretty hard to get you to subscribe for the full service.
There are also good military history search tools (click on Advanced Search) at the site for information on different services (Army, Air Force, Marines, etc), or individual corps, regiments, units, or conflicts.
Earlier posts here at Free Genealogy Tools have covered other military databases in the UK that are good family history resources, including the Debt of Honour Register (WWI and WWII deaths), and, stepping back a few centuries, Soldiers and Knights from the 14th and 15th centuries.
Don't forget to also check for your family history at NewspaperArchive and Newspapers.com. These are subscription databases, but they are among the most powerful research tools available for looking into family roots. And visit the main page of Free Genealogy Tools for more, umm, free genealogical tools.