The Agreement was valid only in case of an aggression, "any military action by Poland" was out of a question:
Actually, "aggression" was defined as both "direct" and "indirect" aggression. "indirect" aggression was defined as any action that changed the status of Danzig that Poland did not like.
If the Danzig Senate had unilaterally declared reunification with Germany, and Poland had claimed that that action "threatened its independence" and sent its armed forces into the city to suppress it, that use by Poland of its armed forces would have triggered the agreement with Britain, obliging Britain to take military action against Germany.
Furthermore, the agreement signed at 17:10 on 25 August 1939 what not something new, it was merely a formalisation of the agreement reached on 6 April, during Beck's visit to London. Here is the relevant excerpt from that agreement of 6 April:
SUMMARY OF CONCLUSIONS
1. As a result of the conversations held in London on the 4th-6th April 1939, between the Polish Foreign Minister on the one side and the Prime Minister and the Secretary of State for Foreign Affairs on the other, the Polish Government and His Majesty’s Government in the United Kingdom record the following conclusions:
2. The Polish Government and His Majesty’s Government in the United Kingdom have decided to place their collaboration on a permanent basis by the exchange of reciprocal assurances of assistance. They are accordingly prepared to enter into a formal agreement on the following basis:
(a) If Germany attacks Poland His Majesty’s Government in the United Kingdom will at once come to the help of Poland.
(b) If Germany attempts to undermine the independence of Poland by process of economic penetration or in any other way, His Majesty’s Government in the United Kingdom will support Poland in resistance to such attempts. If Germany then attacks Poland, the provisions of paragraph (a) above will apply. In the event of other action by Germany which clearly threatened Polish independence, and was of such a nature that the Polish Government considered it vital to resist it with their national forces, His Majesty’s Government would at once come to the help of Poland.
(c) Reciprocally, Poland gives corresponding assurances to the United Kingdom.
(d) It is understood that the Polish Government and His Majesty’s Government in the United Kingdom will keep each other fully and promptly informed of any developments threatening the independence of either country.
3. As an earnest of their intention to enter into a formal agreement to render assistance to Poland in the circumstances contemplated above, His Majesty’s Government in the United Kingdom have informed the Polish Government, and have stated publicly, that during the period required for the conclusion of the formal agreement outlined in paragraph 2 above, 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 the support in their power.
Note that the words used by the British Government are "at once come to the help of Poland", and this action of coming to the "help" of Poland would occur under two circumstances:
1. if Germany attacked Poland.
2. If Germany undertook "other action...... which clearly threatened Polish independence, and was of such a nature that the Polish Government considered it vital to resist it with their national forces".
In the first case, an attack by Germany on Poland, it is clear that "coming to the help of Poland" means that Britain would use its armed forces to assist Poland in fighting against the German forces that had attacked Polish territory. It follows logically that "coming to the help of Poland" in the case of other action by Germany, not involving an attack on Poland but to which Poland had responded by using its armed forces, means that Britain would deploy its own armed forces to assist the Polish armed forces against Germany.