It will probably take several years, but it will be a decisive victory for the continental powers. The naval geography and operations is far more in their favor, since the British have to keep superior forces at home to respond to any threat, and that leaves them uncovered elsewhere : there are certain regions where they simply can't keep covered, such as the Mediterranean, and geography, diplomacy, and the simple balance of naval forces means that proposals like trying to turn that into an advantage by defenses at Gibraltar and in Egypt are either impossible due to neutralization or invasion. Historically it was hard enough to keep the Germans bottled up, now you propose throwing the rest of the European navies in with them (and subtracting the British from their counter), and giving them infinitely better naval geography to boot?
It'll be a grinding campaign of attrition, but the position and strength of the continental nations means that they hold the trump card in that they can strike at the British while the British can't strike at them effectively, meaning that the British have to win every battle, while the continental nations only have to not lose, and even if they do lose they have greater naval construction capacity anyway and hence greater possibilities to take losses. Combine that with greater naval construction capability, armies which pose credible threats (armies are actually a very serious issue in naval battle for ensuring security of naval bases and force projection, I would advise reading for example
The War in the Mediterranean 1803-1810 as an example of its context in the Napoleonic period) and above all else that the continental nations can choose when and where they want to fight (and given that naval operations generally operate on the 1/3 or 1/2 rule, in that only 1/3 to 1/2 of ships can be on station at any one time) that means effective crushing numerical superiority in any engagement they meet in, the picture looks even more glum for the English. Eventually British forces are ground down, control over the sea starts to collapse, the British economy buckles, and it enters a negative feedback loop which only has one outcome.
It looks a lot like a re-run of the Napoleonic Wars. The Royal Navy was slaughtered by the Dutch, French and Spanish Navies and the population of the British Isles had died of hunger by 1815. Not!
The European navies of 1914 are a far stronger force relative to the British than just the French navy of 1792 was, or even the Franco-Spanish-Dutch forces of ~1800 (a time when it must be noted, the British had been temporarily forced to abandon the Mediterranean upon the Spanish entrance into the war, and it took years to regain it) both in quantitative and qualitative terms, changed naval technology favors the opposition to the British (the submarine of course, but also far more powerful coastal artillery which is going to be mostly to the advantage of the Continental nations due to their, well, continental status, mines, torpedoes, aircraft, and naval propulsion technology which relies upon something other than wind and waves and hence which makes hunting down commerce far easier), Britain is much more dependent on trade and hence much more vulnerable to naval disruption, naval geography is significantly improved for the Continental powers since so much of British trade passes through the Mediterranean which is a lost cause for them, and since there is far more terrain available both in Europe and around the world where naval operations take place and threaten the British. In almost every way, the situation is drastically more in favor of the continental nations than it was for Napoleon.
The 1815 British might have been quite content and well-fed, not so sure about their 1915 counterparts.
The won't starve. A series of studies and experiments during WWII showed that Britain was capable of feeding itself in the case of a total blockade. Everybody will be eating potatoes and cabbage for years but they won't starve.
They don't need to starve, it'll just destroy their economy to the extent that their ability to actually prosecute the war will be meaningless.
Britain. The dominions can easily take over colonies, meaning realistically the map would start to look like this early on:
With Namibia, Madagascar, Dutch Indonesia, German Pacific and Indochina falling quickly. After that I think Britain has a fighting chance.
The normal utter nonsense parroted by the British side in such threads : colonies very often fall neither quickly nor readily. It took nearly a year for Namibia, with a tiny population, to be conquered by South Africa (due in large part to South African internal difficulties, but those would be present in the same scenario), Cameroon a year and a half when it was surrounded on all sides by France, Britain, and Belgium and entirely cut off from the homeland, and Tanganyika never was completely conquered. The same during the French Revolutionary Wars : it took until 1809/1810 for the French Caribbean to be taken, and 1811 for Mauritius. Sometimes colonies do fall rapidly, such as German New Guinea or Togo, but most of the time they are not quick and rapid campaigns : it takes years. And that mediocre performance in WW1 was when the Central Powers were almost entirely restricted from maritime activities and their colonies were utterly isolated. Here, they are anything but isolated (such as the Dutch East Indies and the French Pacific colonies being able to assist German New Guinea, or Belgian and German territories to draw attention from Namibia), the British and allies have a lot more targets to attack, less allies, and far, far more enemies and far more tenuous lines of communication. And in that time, a lot of British colonies are going to be facing invasion, like the Western African ones, the Mediterranean, Egypt, which is going to be drawing resources away from going on the offensive.
This is not even mentioning that the loss of places like Namibia has not even the slightest impact on the war's outcome...