The reason why the invasion of Poland caused the UK and France to declare war had little to do with Poland itself. In fact, the UK and France did nothing in all practical terms to defend it. Beyond this, there was hardly any fighting between the Allies and Germany until Germany invaded the Benelux. Both France and the UK were expecting the war to go similarly to WWI, and while they did mobilize their military, they dug in for a defensive trench war rather than go on the offensive, let alone do anything to try and 'save' Poland. (For what it's worth: recall that the USSR was also invading Poland at the time. Given geographical restrictions, as well as militaristic comparisons, it's reasonable to say that there was little the allies could do to save Poland without invading Germany)
To specifically answer your question, the reason why Poland's invasion instigated declarations of war is more that it could be seen as 'the straw the broke the camel's back' in a long list of promises Hitler had broken. Since the Treaty of Versailles, Hitler had broken its regulations in several ways:
Keep in mind that after each of these steps, Hitler was in consistent communication with the leaders of the allies promising he would stop further transgressions.