With no PoDs prior to January 1915, how, by the end of the year, can the Britain successfully execute the Gallipoli Campaign -- capturing Constantinople and/or force the Ottomans out of the war? How does this affect the war? And how is the Middle East affected by its empire being defeated this early in this way? Are the Ottomans more likely to hold together, or are they still carved up a la Versailles?
A lot depends on the POD. Exactly the same resources doing a vastly better job looks different than more resources winning more battles. Also, the level of casualties and consumption of equipment and supplies matters. So I will try to split it into categories.
IMO, OTL had little chance of winning. The initial use of what are really surplus capital ships that don't even need to be replaced when lost is a ok gamble. High risk, but the UK was betting what it could easily afford to lose. The landing is an entirely different matter. The Entente (UK) easiest to supply logistical battlefield is Flanders. Instead the Entente attacks where it is very hard for the Entente to supply. The Ottomans have a lot of troops, but difficulty moving them to the borders. The attack also fixes this problem for the Ottomans by attacking a troop concentration. And to cap it off, into good defensive terrain. So what did the plan need. It was in the plan, but taken out. You need to attack the Asian side of the straights with at least as many troops (12-16 division). So you are pulling a full army out of Flanders. You will not be unloading at French ports but on beaches. You have to move the supplies not from England to France but all the way to Turkey. This means you are not using the ships to move other goods. So not only are you missing a full army in the Western Front, but the remaining forces are missing a lot of supplies.
So lets assume you do get both sides of the straights by 12/31/1915. You might knock the Ottomans out of the war. Might change Bulgaria's actions. But likely Turkey fights on, remember IOTL, the Turks alone force the UK into a negotiated peace, in 1923. But you have also broken Russia. You will not have the attack that forces the Germans to move 330 battalions west in August. The Germans can keep advancing until the snows come. The Russians had retreated for 90 straight days, and likely keep retreating. It is actually a great TL if someone wants to write it.
Now we get some interesting issues related to how much of what Falkenhayn said post war was really what he thought in 1915 and how much is CYA. And what the Kaiser does and other leaders. If the Western Front is easily stable, and Russia is retreating at 30-50 miles or so per week, do the Germans stop at some point in a good campaign season (possibly true) or do they push until winter then decide if Russia or France is easier to knock out of the war (more likely true).
Spliting post.