every person killed while in service; the surviving unit war diaries (most of the Army's; only a few for Aviation) and carnets de comptabilité (a kind of quarterly muster roll); a partial index of aviation personnel; and a list of digitised regimental histories. It is entirely free.
Some of these have been available for some time now, but now benefits from enhanced usability. For example, it is now possible to search the Morts Pour La France database, not only by name (as used to be the case), but now also by date of death, place of birth, and unit. This opens up a large number of possibilities for studying the impact of the war on individual communities, particularly when combined with the results of the 1911 census, and on individual units over the course of the war.
Just to test it out, I looked at those who had been killed, and who had given their country of birth as somewhere in the UK. You pick the country from a drop-down menu, so there are separate searches for Angleterre, Ecosse, Irlande and Royaume-Uni (nothing for Pays de Galles).
William ALLEN, b.7 Nov 1882 Bathampton. Enlisted in Bordeaux in 1916, and served with the Régiment de Marche de la Légion Etrangère (RMLE). Posted missing 17 April 1917 at Auberive-sur-Suippe. He has no known grave.
Alfred Theodore COOKE, b.6 Oct 1884 Stockton-on-Tees. Enlisted in Paris in 1914, and served with the 3e Régiment de Marche de la 1er Régiment Etranger. Died of illness 26 Jan 1915 at the Hospital Complementaire No.41, La Flèche. He is buried in the Carré Militaire, La Flèche.
Corporal George PHILLIPS, b.16 March 1889, London. Enlisted in Paris in 1914, and served with the RMLE. Killed in action 17 April 1917 in the Auberives sector. He is commemorated at the Nécropole Nationale at Bois-du-Puits, Aubérive.
Robert POTTER, b.21 October 1883, Oxted. Enlisted near Paris in 1914, and served with the 2e Régiment de Marche de la 2e Régiment Etranger. Killed in action 11 November 1914 at Blanc Sablon (Aisne). He has no known grave.
Robert POWELL, b.30 October 1892, London. Enlisted in Paris in 1914, and served with the Legion Battalion of the 1er Régiment de Marche d'Afrique. Died of his wounds 27 March 1917 at the Temporary Hospital at Florina (Greece). He has no known grave.
Robert William BACKHOUSE, 172nd Infantry; b.12 Sept 1883 Sheriff Hutton, Yorkshire; class of 1903, enlisted Versailles; died of illness 4 Oct 1918. He is buried in the Nécropole Nationale Friscati, at Vitrimont.
William Stuart Northcote JOHNSON, 158th Infantry; b 27 Jan 1880 Torquay; class of 1900, enlisted Tunis; KIA 8 October 1914 La Cité Saint Auguste (Pas de Calais). He has no known grave.
Captain Edward JORDAN, 29th Infantry; b.31 Jan 1872, Hulnes, co. Lancaster [sic, I think it is Hulme, near Manchester]; class of 1891, enlisted Oran; KIA 20 Aug 1914 Sarrebourg. He is commemorated at the Nécropole Nationale at Buhl-Lorraine, Sarrebourg.
Sergeant John LAWTON, 170th Infantry; b.3 Jan 1887, Stockport; class of 1907, enlisted Epinal; KIA 22 May 1915 Notre Dame de Lorette. He has no known grave.
Robert Clair PHILLIPSON, 404th Infantry (formerly 8e Compagnie des Ouvriers d'Administration); b.30 Oct 1883, London; class of 1908, enlisted Dijon; KIA Mille Kruis sector, Belgium. He is buried in the Cimitière Militaire, Poperinghe-Lyssenthoek. His name is incorrectly indexed as Phillipon on the official French war graves site.
Charles Olive PULLER, 13th Dragoons; b.13 Feb 1886, Wolverhampton; class of 1906, enlisted Versailles; died of wounds 18 October 1918, Belgian ambulance, Vinghem.
George Brenton SILK, 2nd Zouaves; b.31 Mar 1865 [sic - he was fifty!], Harsfort [sic - it was actually Hertford]; class of 1885, enlisted Algiers; died of wounds 25 April 1915, Poperinghe. He is buried in the Carré Militaire, Ypres
Scottish entries include only one real possibility:
Robert Anderson MONTADOR, 31st Colonial Infantry; b.20 Sept 1882, Cellardeck [sic, for Cellardyke in Fife]; class of 1902, enlisted Saint-Omer; KIA 28 Sept 1915, Massiges (Marne)
There is only one Irish entry:
John Joseph BARRETT, RMLE; b.2 May 1890, Ennis, Co. Clare; enlisted 1914, Dunkerque; KIA 20 April 1917, Auberive (Marne)
Unfortunately - there's always an unfortunately, isn't there? - the indexing appears to be incomplete. Searching separately under 'Régiment de Marche de la Légion Etrangère' does not retrieve Allen nor Phillips; '1er Régiment Etranger' does not retrieve Cooke; and '2e Régiment Etranger' does not retrieve Potter. So, a potentially useful tool, but one which has yet to reach its full potential. A page on the site asks for people to get in touch to participate in a collaborative indexing project, so over to you, if you are of a mind to help.
Photographs: the RMLE parade at Versailles in 1918 (Wikipedia); one of the Legion regiments at rest in 1915; a pause for the RMLE during the Chemin des Dames offensive in 1917; Légionnaire Vincent Aich, who appears to have survived the war (all three taken from the site of the Amicale des Anciens de la Légion Etrangère de Paris here).
EDIT: I am grateful to Sophie Pigott, one of the indexers of the site, for informing me that some 'Royaume-Uni' soldiers are now correctly indexed under Jersey and Guernsey (Guernesey in French), which makes them genuine subjects of the Sovereign, even if they are not strictly British. If you do have the time, you should certainly consider volunteering to help with the indexing.
Simce doing the blog page, I have been able to check some of the entries of a well-known subscription genealogy site, and some of these men appear to be in the UK before the outbreak of war, when their fiche implies they were serving in the French Army. I am coming to the tentative conclusion that the 'Class of ...' entry for these men is theoretical only - it represents the conscription class to which they would have belonged had they been native Frenchmen. I have been able to resolve some of the queries about places of birth, etc. (Jordan and Silk, for example). I have also corrected some typos.
SECOND EDIT: keep in touch with the indexing effort by following 1 Jour - 1 Poilu on Twitter @1J1Poilu