It was protecting British national interests

At the time of the First World War and the peace settlement negotiations that commenced in 1919, Britain's main rival for dominance in the Middle East was France; Britain had to continue to support the aims of Zionism in order to persuade France that they were best placed to take control of Palestine. Without Zionism Britain's claims to Palestine were weaker than those of France. Woodrow Wilson, the president of the US, hoped that the two foremost European powers, Britain and France, would support his ideas for self-determination for all nations, but in reality neither Britain nor France had any intention of departing from their rivalry for power and influence whenever the opportunity arose. Britain, therefore, had to pursue dominance in the Mediterranean and the Middle East to protect their position as the foremost empire of the day. Politicians believed that this was the best way to safeguard Britain's prosperity.

Lloyd George was determined to extend Britain’s control in the Middle East beyond the confines of the Sykes-Picot agreement.  He believed that Palestine was essential to the protection of the Suez Canal, and that Haifa, in northern Palestine, was a valuable Mediterranean port. Control of Mosul in Mesopotamia (Iraq) would secure for Britain a vital supply of oil for its naval fleet.

Their open and declared support for Zionsim through the Balfour Declaration provided the British with a strong argument for taking control of Palestine in counter to any French claims.

In 1922 there was a change of government in Britain, and the new Cabinet looked again at its commitments in Palestine. Despite some reservations the Cabinet believed that Britain was honour-bound to keep to the terms of the Mandate. In addition, the creation of the Jewish National Home was an integral part of the Palestine Mandate's stated purpose, so subsequent British governments feared that a retreat from their Zionist commitments  could result in the League of Nations removing their right to govern Palestine, and another power taking on the Mandate. The British were determined that no other power could hold  territory so close to the Suez canal.

At San Remo in April 1920 Prime Minister Lloyd George persuaded the Principal Allied powers (Britain, France, Italy, Japan) to accept the Balfour Declaration as an essential part of the Mandate for Palestine. The establishment of the Jewish ‘National Home’ was now the internationally sanctioned aim of the Palestine Mandate, even though there was no agreed definition of what a ‘National Home’ might be.

This adoption of Britain's expression of support for Zionism as the primary purpose of the Mandate for Palestine, was a triumph for Chaim Weizmann. He had persuaded the British that their best interests lay in making common cause with the Zionist movement.

Map of British and French empires in 1920

The British Empire in 1921