Nice theory, but when you get to the "we now know" part, it makes me question if you understand what "know" means. Much of what you said might be true, but we most certainly don't "know" any of it. Anything that follows from the premise "if we had done (something other than what we did)" is necessarily speculation. The only question is how good the speculation is...
The fact of the matter is, Britain did not get into the war to "try to help the French (and Poland)". They did it to try to save themselves. Whether it was necessary or not is unknown, but nation-states aren't known for committing to expensive (in both lives and money) tasks for altruistic reasons. Britain did what it did because it thought that was what was best for Britain, full stop. If anyone else was helped, that's nice, but not the reason why it was done.