ANKARA, Turkey — Israel's May 31 interception of a Gaza-bound aid flotilla was "completely unacceptable," British Prime Minister David Cameron said Tuesday during an address to Turkish businessmen. In a reference to Israel's blockade of the Strip, he said: "Gaza cannot and must not be allowed to remain a prison camp."He also commented that the world needs Turkey's help in pushing Iran to address international concerns about its suspected nuclear weapons program.Cameron went on to declare that Britain was a staunch backer of Turkey's troubled bid to join the European Union.