The enormous impact Weizmann had made became very helpful for influencing senior administration officials in London. Thanks to his position in the military industry he was close to Winston Churchill, who was the First Lord of the Admiralty, resigned and later appointed at end of the war as Minister of Munitions and David Lloyd George, who was Minister of Munitions and in 1916 was elected prime minister. Weizmann recruited to the Zionist cause Herbert Samuel, the Jewish Home Secretary, who later became the first British High Commissioner of Palestine. In July 1917, Weizmann worded the proposal of the statement, along with Nahum Sokolow, and submitted it to the British government.

Three months later, the government approved a limited and softened version of the proposal, which still contained the promise the Zionist movement longed for - the recognition of the right of Jewish people to establish their national home in the land of Israel.

When asked by UK Foreign Secretary Arthur Balfour what he wished to receive in return for his contribution to the war effort, Dr. Weizmann boldly replied, "There is only one thing I want: a national home for my people." Duly impressed, Lord Balfour issued the famous Balfour Declaration of 1917.

However:

But the suggestion that Palestine was his reward was a figment of Lloyd George's imagination. As Weizmann commented:

I almost wish it had been as simple as that, and that I had never known the heartbreak, the drudgery which preceded the Declaration. But history does not deal in Aladdin's lamps.[3]

A legend has grown up because of what Lloyd George wrote in his autobiography, that the Balfour declaration was a reward for Weizmann's work in biochemistry for the Admiralty.

...

But the suggestion that Palestine was his reward was a figment of Lloyd George's imagination. As Weizmann commented:

I almost wish it had been as simple as that, and that I had never known the heartbreak, the drudgery which preceded the Declaration. But history does not deal in Aladdin's lamps.

At least it is so if this website can be trusted: http://www.balfourproject.org/427/

Looks serious enough. But when it comes to Israel and Palestine questions there is so much disinformation floating around, hence the disclaimer.