Poland had a guarantee from Britain that would only be effective if Germany committed an act of aggression against Poland
Yes, but what constituted an act of aggression against Poland?
The wording of the guarantee left it entirely to Poland to decide whether a particular act by another state (= clearly understood to be Germany) threatened its independence.
Again, the wording of the guarantee meant that the crucial factor that would impel Britain into war against Germany was not the nature of the act, but whether Poland sent its armed forces into action in response to that act.
Observers at the time, and historians subsequently, concluded that the guarantee had placed in the hands of the Polish Government the decision on whether Britain would make war on Germany.
From the very beginning it was understood by the British Government that the act by Germany which would impel Britain into war against Germany would occur in Danzig. The British political and military leaders did not think that the war would begin with a full-scale German assault on Poland; rather they thought that Poland would respond to an incident in Danzig, after which fighting between German and Polish forced would gradually escalate, culminating in a full-scale armed conflict that Poland would lose after a few months. That is the reason why the British military leaders thought that the German-Polish conflict would keep German forces tied down in the East for several months before Poland was finally overcome.
That an incident in Danzig rather than a German invasion of Poland was seen as the likely trigger for war is shown by Chamberlain's actions immediately after the issuing of the guarantee of 30 March. He arranged for "The Times" to publish an analysis of the guarantee in which it was stated that it did not apply to the territory of Danzig, only to sovereign territory of Poland itself. That was clearly an attempt by Chamberlain to limit the open-ended nature of the guarantee, to recall the "blank cheque", to prevent Britain being forced into war against Germany by an incident in Danzig; if he could succeed in excluding that possibility, a war with Germany would only ensue if Germany actually invaded Poland itself, which Chamberlain thought unlikely, since he considered that Hitler would not risk war with Britain.
Unfortunately, the "Times" article elicited a violent response from the War Party in the British political establishment, which demanded that the Government issue a statement that the guarantee applied to the territory of Danzig just as much as to the territory of Poland itself. In the face of the public storm whipped up by the War Party, Chamberlain did issue a statement confirming that the guarantee applied to Danzig.
What that meant was that the Danzig issue could not now be solved by a coup d'etat in Danzig, reuniting the Free City with the Reich, since that would trigger war if Poland resisted with the use of its armed forces. Hitler had thought that Beck, due to internal opposition from the National Democrats and other anti-Sanacja elements, could not be seen to agree to the German requests for the reuinfication of Danzig with the Reich, but that he would accept a reunification by coup, which would effectively remove the whole Danzig issue from the agenda. To that end, the Wehrmacht already had a plan for the occupation of Danzig only, the preparation of which Hitler had ordered on 28 November 1938, after the Polish and Soviet Governments had publicly reaffirmed their Non-Aggression Pact of 1932.
It was most probably the confirmation by the British Government that the guarantee of 30 March also applied to any German action in Danzig that caused Hitler to decide to order the preparation of a contingency plan for a full-scale attack on Poland to eliminate it as Britain's eastern front completing the encirclement of Germany.
It is highly significant that this new contingency plan, "Fall Weiss", did not replace the plan for the occupation of Danzig only; both plans continued in force, as laternatives to each other. That fact suggests that at the time the order for "Fall Weiss" was given, the beginning of April, Hitler had not yet definitely decided on a full-scale invasion of Poland, but was keeping his options open, with the option for an occupation of Danzig only still possible if Britain could be persuaded not to use such as occupation as a casus belli.
It is not clear when exactly Hitler made a definite decision to launch "Fall Weiss".
To avoid war, a reasonable German statesman (who could have avoided Britain giving the guarantee to Poland in the first place) would have continued negotiations, perhaps trying to work through a concert of Europe and played on the not inconsiderable sympathy in the west for the German position on Danzig.
That assumes that Poland was led by reasonable statesmen.
Unfortunately, the heirs of Pilsudski were weak and unable to resist the pressure from the anti-Pilsudkite opposition in Poland, which wanted war with Germany in order to seize the German territories east of the Oder-Neisse Line.
The right-wing Polish nationalists, in particular the National Democrats (Endecja) had been dissatisfied with the Treaty of Versailles, which had not given Poland all the German territory which they claimed on the basis that in the 10th Century it had formed part of the Piast state. Until Pilsudski's seizure of power in 1926, they constantly pressed for armed action to seize those territories, which would have been feasible since at that time the Polish armed forces were much stronger than the feeble forces allowed to Germany under the Treaty of Versailles. It was only the opposition of Britain that prevented Poland invading Germany under one excuse or another.
When French forces occupied the Ruhr in March 1923, on the pretext that Germany was not fulfilling its treaty obligations, the French Government actually invited Poland to invade from the East. However, firm opposition by Britain prevented such an action.
When Pilksudski seized power in 1926, he suppressed Endecja and other anti-German elements, and no invasion of Germany ensued. Particularly after Hitler came to power and initiated the policy of German-Polish detente, all anti-German activity in Poland was suppressed.
However, after Pilsudski's death in May 1935, there was a resurgence of the anti-German elements, which used the struggle for power between the rival heirs Beck and Rydz-Smigly to influence the Sanacja regime to adopt its policies. The anti-German elements once more began to call for war against Germany to seize its eastern territories; maps were published and widely circulated showing the Polish western frontier running just to the east of Berlin, and incorporating almost all the Baltic coast of Germany. (Interestingly, those maps also showed all of Czechia and Slovakia as Polish territory).
Of course, once Germany began rearming, it was no longer possible for Poland to conquer German territory on its own; that end could now only be achieved through a coalition war against Germany, in alliance with France and possibly also Britain. It was the British guarantee of 30 March, followed by the Anglo-British Agreement on Mutual Assistance of 6 April, that opened the clear possibility of such a coalition war against Germany and the conquest of German eastern territory.
It is significant that throughout the war hysteria that was whipped up in Poland in the summer of 1939 by oppositional elements, unhindered by the Polish Government, showed no sign of any fear that Poland was about to be invaded by a powerful enemy and needed to be defended by powerful friends. Rather, the public mood engendered by the war hysteria was that the time had come to crush Germany and seize its eastern territories; there were public calls by the leaders of the Polish opposition for the Polish Army to take the initiative and march into East Prussia and Silesia, and the Polish military leaders boasted that the Polish cavalry would ride into Berlin within a fortnight of the commencement of hostilities.
The Polish keaders were of course banking on a French offensive in the West that would crush Germany. That offensive never came, the Polish Army was crushed, the leaders of the sanacja regime fled into disgraced exile and were replaced by a Government-in-Exile consisting of members of the anti-Pilsudskite opposition.
It is significant that the Polish Government-in-Exile almost immediately raised with its British and French allies the proposal that the Oder-Neisse Line be recognised as the future Polish western border, and that German territories east of that line be ceded to Poland and the ethnic German population expelled. However, Britain did not accept that proposal until 1943.