There were only a handful of Jews in Palestine before the European Zionists arrived. Even after decades of invasion Jews represented no more than 10% of the population when the Mandate was established.
"The Jewish element of the population numbers 76,000. Almost all have entered Palestine during the last 40 years. Prior to 1850 there were in the country only a handful of Jews. In the following 30 years a few hundreds came to Palestine....."
Mandate for Palestine - Interim report of the Mandatory to the League of Nations/Balfour Declaration text (30 July 1921)
It does not matter whether the land was sovereign or not. The Covenant of the LON, article 22, stated clearly that the inhabitants were to be tutored towards self-determination. Clearly, the British neglected to provide tutelage to 90% of the inhabitants and provided it to recent arrivals from Europe and non-inhabitants still in Europe.
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ARTICLE 22.
To those colonies and territories which as a consequence of the late war have ceased to be under the sovereignty of the States which formerly governed them
and which are inhabited by peoples not yet able to stand by themselves under the strenuous conditions of the modern world, there should be applied the principle that the well-being and development of such peoples form a sacred trust of civilisation and that securities for the performance of this trust should be embodied in this Covenant.
The best method of giving practical effect to this principle is that the tutelage of such peoples should be entrusted to advanced nations who by reason of their resources, their experience or their geographical position can best undertake this responsibility, and who are willing to accept it, and that this tutelage should be exercised by them as Mandatories on behalf of the League."
Avalon Project - The Covenant of the League of Nations