Great Britain enter World War I in 1914, their prime minister at the time was David Lloyd George. At first Great Britain only gave diplomatic help to Serbia along with France, it was until the start of the great war (1914) that these countries started giving military support to Serbia who was at war with Austria-Hungary who had the support of Germany and the other members of the Central Powers conformed by Ottoman Empire (currently Turkey), Italy at the start and Germany in destroying Serbia. Great Britain did a great support mostly on naval warfare, which was almost invincible, by fighting the Germans. Great Britain suffered a lot for participating in this war because in the end, the war concluded in a period called the Great Depression.
Great Britain in the Russian Revolution:
They participate helping Russia against the Communists, joining the white army and helping fighting vs the Red Army, composed of Lenin and the communists, but at the end the white army losed, and Great Britain lost the alliance they have and the relationship of the WW1.
http://www.bbc.co.uk/history/historic_figures/george_david_lloyd.shtml
The Russian Revolution: Sheila Fitzpatrick – DK 265
World War 1: Neil M. Heyman - D 521