they AUTHORISED ???
They publicly stated their intention . On the basis of which they received their mandate.
I will repeat.
Lord Robert Cecil , The Deputy Foreign Secretary representing the British Government on 2 December 1917 stated that the British Government fully intended that “Judea [was] for the Jews. Our wish is that Arabian countries shall be for the Arabs, Armenia for the Armenians and Judaea for the Jews”.
The San Remo Resolution adopted on 25 April 1920 incorporated the Balfour Declaration of 1917. It and Article 22 of the Covenant of the League of Nations were the basic documents upon which the British Mandate for Palestine was constructed.
All AFTER they were in possession of the Land.
It is true that the British themselves backtracked afterwards. And that subsequently some Zionists fought against them.
But the League of Nations ,under whose mandate the British governed ,never backtracked.