the law of the Polish Second Republic was quite clear on the issue of a citizen that enlist to a foreign army
Have you actually read the statute? Roman Wisniewski, a Polish lawyer in Warsaw, has posted an English translation online with a detailed explanation of the "military paradox" here:
polishcitizenship.pl/law/
Do you disagree with his translation of the last sentence (or two depending on the punctuation) of Article 11 of the Polish Citizenship Act of 1920?
See here:
cklawoffice.eu/polish-citizenship-law.html#1
(I would quote it, but that seems to be against the rules here for some reason.) Unlike you, he is a practicing lawyer in Poland.
The Second Polish Republic was a much different place than what followed. It was founded in no small part by 20,000 to 25,000 Polish immigrants in North America who returned to fight for Poland's independence first in France under Gen. Haller, and then in Poland against the Bolsheviks. Criticized by its detractors as overly nationalistic, and attempting to create a "Poland for Poles", it was eager to have those who left the partitions return as citizens. The final wording of Article 11 was in there for a reason. They wanted people living in the West to return, and serving in a foreign army was no barrier to continued citizenship under certain circumstances, i.e., permission from the government, or failure to serve first in Poland's military, etc. The Third Republic was not the Second. The Third Republic was happy for trouble makers to leave, so immigration served as a release valve for political problems, i.e., like expelling the Jews.
my parents remained in the UK after the war, father had served in the British army for eight years.
Was he in Anders' Army? That has gotten reclassified in recent years as a Polish expeditionary army even though it was under foreign command. The commies, of course, considered it a foreign army. They rightfully feared that Anders might lead it against them.