Here are useful links that you can use to start your search:

Free Websites

Commonwealth War Graves Commission : www.cwgc.org

War Memorials Trust : www.warmemorials.org

UK National Inventory of War Memorials : http://www.greatwar.co.uk/organizations/ukniwm.htm

Great War 1914-1918 : www.greatwar.co.uk

Imperial War Museum : www.iwm.org.uk

RAF Museum Hendon : www.rafmuseum.org.uk

Death Certificates : www.direct.gov.uk

Family Search (Mormons) : www.familysearch.org

Lives of the WW1 : https://livesofthefirstworldwar.org

National Archives : http://www.nationalarchives.gov.uk

London Gazette https://www.thegazette.co.uk

London War Memorial www.londonwarmemorial.co.uk

Subscription sites:

pending