mlees,
I can’t help but wonder if England didn’t want Turkey to join the Central Powers. England liked to snap up territory at the periphery.
Nawwww…to paranoid 
but I’ve wondered.
Also, I’m surprised at the number of people that think that Turkey sucked in WWI. From what I’ve read, it seems (to me) that Turkey did pretty well, considering the circumstances.
In addition to Germany’s unrestricted submarine warfare, which killed a number of American civilians and inflamed public opinion, the Zimmermann Telegram was a big reason for the U.S. declaration of war. (See Zimmermann Telegram - Wikipedia). And having entered the war, Wilson and the U.S. military leadership were not going to limit themselves to naval involvement when the British and French armies were so hard-pressed on the Western Front.
I suppose it’s based on the outcome, which included the permanent and irretrievable dissolution of the Ottoman Empire.
Returning to the OP "why jump into the war at all? " most of the answers have already been covered but it is worth pointing out that it was a close run thing as to whether Britain would enter or not and, if it did, whether the major part of the Army would fight alongside the French in northern France.
On the 3rd August 1914 the Liberal Government was split over the response to the gathering crisis. Eventually Grey, the Foreign Secretary, brought the Cabinet round but there were resignations and threats of resignations before it was settled. Grey set out Britain’s war aims in a speech to the Commons on the 3rd August as protection of the northern coast of France, preserving Belgium neutrality, and preserving the balance of power.
Defending the French coast was a necessity because, to meet the threat of the High Seas Fleet, Britain had withdrawn the Mediterranean Fleet leaving the French to look after its interests - declaring neutrality and leaving the French coast open to attack would have immediately negated this and Italy, a member of the Triple Alliance, might well have threatened Britain’s interests.
Keeping Belgium neutral, specifically avoiding Antwerp falling into enemy hands, had always been a key British objective. The thought of an unfriendly fleet that close to the Channel and the Thames Estuary was deeply unattractive.
Preserving the balance of power, again was a long term policy objective. Whether Louis XIV, Napoleon. or the Kaiser it was clearly not in Britain’s interests to have one power controlling all the resources of Europe.
With all three of these reasons in play it was effectively impossible for Britain to stay out of the conflict - which begs the question of why Germany invaded Belgium. If the much vaunted German General Staff had avoided invading Belgium as part of the idiotic Schlieffen Plan and instead guaranteed its neutrality only two of the three aims would have come into play. In these circumstances it would have been a lot harder for Grey to get agreement in the Cabinet and the country – defending “plucky little Belgium” was an emotional cause around which support could gather.
As to why Britain ended up fighting a continental war, you could say it just happened!. Providing a smallish expeditionary force to support the continental ally was standard operating procedure and, in the pre-War discussions with the French it was felt the six divisions of the BEF would serve to tip the balance between the French and German armies. Worth noting that pre-war the Navy and the Army never really settled how Britain was to fight the war: BEF in northern France or Navy landing small forces on the German coast. As the war approach it became obvious there was no way the Royal Navy could risk major forces near the German coast, certainly not in the Baltic as some optimists claimed, given the risk from mines and torpedoes mentioned up-thread. It was this threat that led to the policy of “Distant Blockade” with the Grand Fleet at Scapa Flow rather than cruising the Heligoland Bite within sight of the coast.
So the BEF was committed on the left of the French line as the Germans swept down through Belgium. Once there and following the staggering French losses of 1914 and early 1915 there was no choice but to build up the Army in support of the French. Remember Britain was very much the junior partner right through until 1917 and strategic direction was in the hands of the French. Given that the Germans were occupying (and exploiting) the key industrial areas of northern France the French were inevitably going to try and drive them out and the British had to go along with it. That meant taking over more of the line with more and more troops.
In the end it took the combined forces of Russia, France, and the British Empire to grind down the German army during 1916/17 sufficiently to allow the advances of 1918 – and even then it was pretty close with the release of German forces from Russia before the AEF could exert anything like its potential strength.