I know that from pretty much until the early Middle Ages until into the colonial era England and France were fierce rivals but there's a gap in my knowledge until they're suddenly allies in WWI which lasts until today. What happened?
The pre-war turning point would be the creation of the Entente Cordiale. At this point, the French and English united against an antagonistic Germany (to simplify it somewhat).
It wasn't a true alliance per se, but there's a lot of evidence to suggest that the moral obligation Grey felt played a large role in the British decision to enter the war on France's side.