I am aware that after WW1, the two countries were a lot closer than they had been previously in many ways. But what was the formal British military position regarding France?
Britain declared war on Germany after the invasion of Poland, however, had Germany not invaded Poland and gone straight for France would Britain have declared war?
I don't have time to write a detailed reply on it, but the best book I ever read on the matter was William L Shirer's "The Collapse of the Third Republic". Shirer is the author of the more famous companion book "The Rise and Fall of the Third Reich" which covers things from the Nazi perspective.
Third Republic spends about a third of the book describing the period from the birth of the Third Republic after the Franco-Prussian war to the end of World War One. The remainder of the book is dedicated to investigating the political, cultural, and economic causes of the French collapse in 1940. As you'd imagine, he goes in depth into their relations with the UK during the period as French and British foreign policy were deeply intertwined (although not uniformly in concert) during the period. I will caution, though I think it's clear to any reader, that he is a bit more personally invested in France than he was as the author of Third Reich.
Edit: Typo. Somehow I managed to type "republic" as "repiblic" twice.