You may be interested in past posts on this topic:

• When did the British-French relationship changed from hated rivals to friendly allies ?
  ^(07 Dec 2014 | 8 comments)
  ^(/u/Jvlivs details the factors pulling the two powers together before the Great War.)

• How did the English and French become allies for World War 1 when they've fought for hundreds of years prior?
  ^(27 Jun 2013 | 8 comments)
  ^(/u/Klarok describes push factors for the relationship.)

• Why were England and France allies during World War I when they were enemies in centuries past?
  ^(03 Oct 2015 | 4 comments)
  ^(/u/When_Doves_Cry focuses one threats to British naval supremacy as a factor in the alliance.)

For followup questions, use username mentions in this thread to direct questions to specific commenters.