"Forces War Records is the definitive location for military genealogy records online. The database includes records from WW2, WW1, the Boer War, the Crimean War and beyond. It is the only online database which now includes over 4 million military records of British Armed Forces personnel exclusively cross matched with over 4000 Regiments, Bases and Ships of the British Armed Forces going back to before 1350."The site is well organized. You can perform a free search using first and/or last names and record type (war). Results returned also indicate the rank and date of the record.
I found a Canadian WW1 serviceman so give it a try for other British Commonwealth country soldiers.
There's a helpful military genealogy tutorial.