First of all go and check Balfort declaration which was the prime document form as the basis of formation of Israel ... moved by Britan ...
Even wiki has info available on that
https://en.wikipedia.org/wiki/Balfour_Declaration
Second part about ownership of land ... Is Palestine is west ??? Why are you applying western concept of ownership of land on east ??? Is it contradiction ???
My friend most of the territory of Pakistan is owned by government it is always like that ... Poor farmers use to grow crop on these government lands and then sell it to earn their living even today ... so 80 years back ownership of land was even more informal ... By your concept if someone occupies Pakistan then on the basis most of the land is owned by gov of Pakistan therefore the occupier automatically owns land legally ... How this is logical or even legal ??? In any case whatever agreement between jews, Britain and ottoman happened it was out of result of direct force... ottoman lands were occupied and they gave their right as they did not had capacity to fight ...
Regarding fair ...In the first place jews are occupier of palestine ... so if they have been oppressor so long so their childs are now even grown old so does this change the reality that jews come in Palestine as occupiers through various treacherous methods ?
The Balfour declaration speaks about a Jewish Homeland, but has no legal implications.
As for the legality of British presence...
For starters, the Ottoman land was only Ottoman, as a result of conquest.
Conquest used to be a perfectly legal way to acquire land.
The British did not
occupy the Mandate in the legal sense, due to the treaty signed after WW1,
where the Ottoman Empire ceeded their claim to the area. That makes the transfer legal.
Thus land owned by the government of the Ottoman Empire also was transferred to Great Britain.
Great Britain ceeded their claim to the Mandate when they left, and Israel claimed the part of the Mandate
within their borders as the successor.
The formation of Israel was after a vote in the UN general assembly, and countries
accept that the UN can make decisions.
Only the UNSC can make binding resolutions, but following the result of a vote in the General Assembly
can hardly be considered illegal.
When Israel was formed, the Arab population did not automatically lose their property.
Any Arab that remained kept their property, only those that left, lost their property which
is perfectly legal according to the Ottoman land law.
Likewise jews that left Arab countries for Israel lost their belongings.
After the formation of Israel, the West Bank was occupied and then annexed by Jordan.
Very few countries except for Pakistan recognized the annexation.
In 1967, Israel occupied the West Bank and Jordan later revoked their annexation, leaving the West Bank in limbo.
Israels Supreme Court assumes that they as occupiers have the right to act like the government,
and applies the land law of the Ottoman Empire, where ownership of unused "Dead" land is transferred
to the government , i.e. Israel.
It is implied that if Israel leaves the area for the formation of a new state, the government land
will be transferred to the new state.
Anyone that starts to use Government land will get ownership of the land (while they are there),
so settlements take ownership, when they form.
Settlements created on land in use are illegal, and have been terminated by the Israeli Supreme Court.
It is illegal to "transfer" your population into occupied territory, according to the Geneva Convention.
What is questionable is whether the Geneva Convention is applicable since it is only applicable between parties
which has signed the convention and the Palestinian authorities did not sign until a few years ago.
There is nothing in the convention which says that it is to be applied retroactively,
so the legality of anything on the West Bank is highly questionable.
While the Palestinians have announced a state it is not recognized by Israel,
and Israel does not neccessarily consider the West Bank as a part of an occupied Palestinian state.
So the transfer of land is according to Ottoman Law.
If Western Law had been in force, then Palestinians owning land would not lose it, if they left.
I repeat that Palestinians have difficulty getting what they want if laws and conventions are followed strictly.
They have to argue for a fair settlement of the conflict.
United Nations 
