I don't know at what stage of the War this planning occurred, but remember that even though Gallipoli failed, the Ottomans still lost the war so we know what the allies actual did--including setting up a semi-puppet regime in Istanbul.

The Greeks did ultimately want Istanbul, but were unable to take it in part because the British (along with other allies) were occupying it. And they were not necessarily eager to give it up to the Greeks. This was done in accordance with the Treaty of Sevres, which had Istanbul under "neutral", mainly British occupation (though with the Sultan still in place as a figurehead) and a Turkish rump state in Anatolia. See the Occupation of Constantinople, for more details. This occupation lasted from 1918 to 1923 and only really ended with the Chanak crisis, where it was clear that the non-puppet Turkish military under Ataturk would go to war and the British would not. The Greeks occupied and were eventually given all the Thracian land right up to what was then the Istanbul suburb of Çatalca, often romanized as the Chatalja line.

The chaos of post-WW1, post-Sevres Anatolia--where Greeks, Soviets, Italians, independent Armenians, the French, and the Republican Turks (lead by Ataturk) were all claiming large chunks of land--seems to indicate that there wasn't a plan so much as many plans that sort of got cobbled together into the 1918 Treaty of Sevres, which the Republican Turks felt went too far and the Megali Idea-loving bloc of Greeks didn't think went far enough.

This map of the results of the Treaty of Sevres should give you a clear idea of the plans that were actually put into place after WW1, which I imagine reflected the planning that occurred before and during the War, as well as post War negation X. Control of the Turkish Straights (the Dardanelles and the Bosphorus, the latter of which Istanbul straddles and the form of which the fight for Galipoli was over) was an unambiguous British war goal. The issue had been on the British strategic radar for three generations at this point, since control of the Straights could control Russian influence in the Mediterranean. I've only really looked at the war from the Turkish and Ottoman perspectives, so I don't know much about allied communications, other than widespread support for an independent Armenia, often called Wilsonian Armenia, after American President Woodrow Wilson (the French even signed an agreement in 1916 supporting an independent Armenia), general support for the continued expansion of Greek territory (though the Italians objected to this), and large European spheres of influence in Anatolia à la the Sykes-Picot agreement. Here's a 1916 map that shows how much of modern Turkey the French claimed in the original Sykes-Picot agreement (notice that the borders changed, especially in modern northern Iraq, I believe because oil was discovered there--it's why both the British and Ataturk wanted to claim Mosul up until the Treaty of Lausanne). Likewise, the Italians was still looking for their "place in the sun" (that is, colonial empire) and "in the event of the partition of Turkey" (i.e. the Ottoman Empire), the Antalya province as early as 1915 was all but promised to them (Generally speaking, France, Great Britain, and Russia recognise that,... in the event of total or partial partition of Turkey in Asia, she [Italy] ought to obtain a just share of the Mediterranean region adjacent to the province of Adalia [Antalya]...). A 1917 agreement expanded this to also include the area around Adana, as well. Italy would later object to Greek post-War claims for Izmir/Smyrna on the grounds that that was supposed to be part of its own sphere of influence.

The post-War negations around Smyrna/Izmir really show how unclear the plans were during the War--the Italians and French saw Anatolia having a much more colonial future, like what ended up happening in the Arab World, and the British and American pushed for "national self-determination", and giving it to the Greeks. The Turks wanted it in their nation state, and the Greeks wanted it for theirs, and some Brits pushed for a "separate administration" for the area (perhaps like occupied Istanbul, perhaps like the short-lived "Free City of Danzig" in Poland). In 1919, it looked as if the Italians and the Greeks (and the Turks) might all fight over Smyrna. Eventually, the allies endorsed a Greek occupation of Smyrna but left open the eventual question of sovereignty until the 1920 Treaty of Sevres. But this incident shows that even if there were plans for what would happen after the War, they were always multiple--even within the British government there were multiple, conflicting ideas about what should happen to Smyrna/Izmir.

So, in short, I can't tell you exactly what the Allies were thinking before and during the War, but we can see very clearly what they did after the War through the Treaty of Sevres, which was pushed by the allies and signed by a mostly puppet Sultan in Istanbul. I should mention Sultan had mostly been a figure head since the Young Turk Revolution of 1908, and therefore the Sultan was seemingly open to the idea of a (temporary) occupation that returned him to power. However, within three years, Sevres was annulled through blood (the Republican Turks under Ataturk routed the French, Armenians, and Greeks, and showed they were willing to go toe to toe with the British) and the 1923 Treaty of Lausanne, which established modern Turkey as a fully independent nation-state.

As a follow-up question, what was the Entente attitude towards Greece?

To add to this here is a flag that I have been told is what the British planned to use for the future Colony of Istanbul. Where can I find more information on this?