Indeed: https://en.wikipedia.org/wiki/British_declaration_of_war_on_.... Both Britain and France declared war on Germany because they made guarantees to Poland about it.
> ... in the event of any action which clearly threatened Polish independence, and which the Polish Government accordingly considered it vital to resist with their national forces, His Majesty's Government would feel themselves bound at once to lend the Polish Government all support in their power.
That is unambiguous and clear. They kept their word.
It is tragic in the end that after the war they handed Poland over to Stalin. Poland still had its independence threatened but after having supplied and helped Stalin all that time, it was awkward having to declare war against him as well.