Has there ever been anything approaching an 80% turnout? General elections are rather important and don't get that much. I think the exit fans will be more committed to voting too.
For the Scottish Referendum numbers hit 85%. In parts of Scotland it was 92%. I feel turnout for this referendum will be just as high.
Based on what I've read there's a slight majority in favor of leaving right now, depending on who you ask. This is also a very large and important vote, so I would expect at least a 70% turnout for this referendum.
For gentlemand, general elections typically have lower turnouts for reasons such as people staying home because their candidate was out, or they live in a dense area of one or the other type of vote, they realize that, and proceed to stay home.