I'm currently in a college history class and we just covered this topic, so I'm going off of lecture notes, I hope that's alright. If not or is someone wants to give better sources that's ok.

Now, as I understand it, after WW1 pretty much all of Europe was devastated. Every country was defaulting on debts, as this circle of waiting for reparations to pay your own debts soon fell apart.

This is important because at this time, the "war to end all wars" had passed, and Europe was largely demilitarized and had made an agreement within the League of Nations to not gear up for war, and to try to prevent a second war.

However, the League of Nations was a joke more or less, if for no other reason than the country who pushed so hard for it's creation (The United States) was not even a part of it, and so these agreements were not widely upheld, most notably as hitler comes to power.

Now I gave you all this context to get a better idea of how Europe was as a whole, and why war was not instantly declared.

So, as Germany began to become militarized once again, we know this pissed off the allies who are still recovering and aren't ready for a second war. As Hitler took power and began occupying different areas, it angered the global community at his totally abhorrent following of the agreements made before.

Despite that, war was hugely unpopular. Massive casualties and crippling debt were fresh in the people's minds, and so when Germany began occupying lands, it was upsetting, but the French and British governments merely condemned hitler, and attempted at having conferences and treaties to basically say "we don't want war, you took plenty of land, now stop"

Eventually hitler overstepped his bounds and war was declared. In other words, Britain and France did what they could to prevent a second war, however as hitler invaded closer to France and other neutral nations (including the Rhineland) this understandably got the French government worried and angry with germany.

I hope this helped give a general idea of how Britain and France felt/why they reacted the way they did! I'm on mobile so if anything is typed oddly or needs clearing up please let me know!