Okay so I've read a bit about WW1 and it seems that British diplomats had no idea what they were doing. At every turn they seemed to be writing checks they couldn't cash or immediately went out and made another agreement that broke the first one. The treaty with Italy and with the pan Arab revolt stick out in my mind. Did they think that they could smooth things out later? Did they just not care? Did the people in London not notice what their agents were doing? Did they not respect the people they were making treaties with and breaking them in the future wouldn't matter as long as they fought against the central powers? After the war were countries less likely to deal with the British Empire because of all the broken promises? Or do I have it entirely wrong at it was a success?
Speaking to the Arab Revolt, the British weren't writing a check they "couldn't cash", they were playing an exceedingly careful game that many would argue was masterfully played. They didn't necessarily expect what followed, but it's more likely they saw the use of these contradictory promises as potentially problematic later, but not as urgent or eventually painful as the loss of WWI.
The British, in attempting to get the Arab revolt going, were looking to weaken the Ottoman Empire to keep the Entente powers going. I can't go too far into detail on the military goals of this, but they included (from my memory) aspirations to get Iraqi oil (the Royal Navy now worked on oil, I believe), get control of the Mediterranean, and otherwise tie down enemy troops.
I'm presuming you are referring to the contradictory promise made in the Balfour Declaration and the McMahon-Husayn correspondences. While some scholars argue that the British had no ill intent and simply didn't see the promises as conflicting, this is a very rare position. In reality, most argue, the British intentionally left the promises vague and twisted them semantically later. They wanted to gain the support of the Arabs, but at the same time didn't want to alienate the Jews. This was the cause of the Balfour Declaration (in addition to lobbying and Jewish contributions to WWI, specifically Weizmann's research with acetone for ammunitions, as I understand it), which was meant to inspire support for the Entente among the Jews. This was particularly hoped-for in Russia, where unrest seemed to be threatening to pull the Russians out of the war. Charles D. Smith argues that the Balfour Declaration was thus meant to prop up the Russians, while the McMahon Correspondences were left vague to keep Britain's options open and give them a semantic "way out" when necessary. Again, the idea this could cause serious troubles was dwarfed by the fear of losing Russia in the war and the want to tie down Ottoman troops while gaining a foothold in the Middle East.
This doesn't suggest poor diplomacy, it suggests a very fine line that the British diplomats managed to walk to ensure WWI victory, while hoping to avoid being tied down. To a certain degree, part of the problem arose also from miscommunication and liberties taken by diplomats involved in the McMahon correspondences, and the instructions given to those diplomats being vague. But at the same time, to suggest the British "didn't know what they were doing" is definitely...not right. In fact, I'd argue British diplomacy was extraordinarily masterful in getting the Arab revolt going with such vague promises, and in getting Jewish support, though it ultimately failed to prevent the Russian revolution. The British acted with clear goals and managed to walk the finest of tightropes to ensure their victory, and I'd say it was impressive, even if it was underhanded or ingenuous.
The biggest failure of British Diplomacy was to not come out and tell Bethman-Hollweg that the UK would fight along side France, if the Germans invaded Belgium. If Gray was more open in his support of France and Russia, the Kaiser might have waffled on his unwavering support for Austria. As it was, Berchtold dithered from June 28 until late July. The Kaiser went on vacation and the situation spun out of control. Gray and Bethman Hollweg did very little to stop the slide towards war between June 28 and July 28 1914.