The EU will make a number of consessions and Cameron will declare it is enough. Then the main parties will all campaign for a Yes vote, as will Business leaders and Union leaders. The yes campaign will be a juggernaut. I expect the result to be close, but that we will stay in. I expect the AB will not be representative of the nation as a whole, because members are mainly old people.
After many conflicts and two world wars the EU was formed to bring the nations of Europe closer together and avoid further wars. There hasn't been another major European war for 70 years. It would be shameful to turn your back on the Union and walk away.