Did England enter the war because Germany defied serbian neutrality?

No. England entered the war due to France entering the war due a defensive pact. France entered the war due to Russia entering a war with Astria-Hungary due to Russia's pledge to defend Serbia, which was being attacked by Austria-Hungary due to the assassination of Franz Ferdinand, the Austrian heir-apparent. Germany supported Austria-Hungary in their war with Serbia which made Russia delcare war on Germany, thus leading France and then England to declare war on Germany. It's important to keep in mind that while all the great powers of the time were responsible for the war. It is particularily onerous on part of German Kaiser Wilhelm II for annulling the treaty with Russia made by Otto Von Bismark (who Wilheml sent into retirement). The treaty assured that if either Germany or Russia were attacked, the other power would declare neutrality. Wilhelm saw this as contrdictory, since Germany was friends with Austria-Hungary, who was in turn a rival with Russia. This shows a poor understanding on Wilhelms part of the concept of the balance of power endorsed by Bismark. Which said that countries must be kept in check through alliances that would ensure that wars would be fought between as little combatants as possible. Bismark also saw the importance of not having a war on two fronts with France and Russia. The latter of which was a long time rival of Germany. It's also good to note that Wilhelm II seemed to have misinterpreted the treaties with Austria-Hungary that said that either country would defend each other if attacked, not if the other guy were doing the attacking, which Austria-Hungary was doing to Serbia. Wilhelm got cocky, and it was going to get rocky.