Absent voters

This week is the last chance to register to vote in the 2012 London mayoral election, assembly elections and (elsewhere) local council elections.  In 1918, a completely new electoral register was created as all men over 21, women over 30, and servicemen over 19 were allowed to vote. This register marks out those who were serving in the armed forces and allows us a snapshot of military service in the summer of 1918 when it was compiled. How many Londoners were serving? How did London compare with other regions?