I don't personally think there will be a mainstream 'out' campaign. I can't think of a major group for whom pushing for out would be in their interest.
And moderates would be revolted by the key figures fronting such a campaign.
I was reading in Sandbrook's 'Seasons in the Sun' that in the 1975 referendum, the 'yes' campaign was led by moderate and popular figures of the major parties, but the exit campaign was fronted by all those considered 'outsiders' such as Enoch Powell and Tony Benn who were likely to deter people.
If the fruitcakes were kept away, a campaign led by, say, the eloquent Giesla Stuart or Dan Hannan might provide the calm reasonableness of the Eurosceptic case, but they'd always risk being tarnished by the actions of the fringe
Course, it might be worth leaving just to annoy the SNP (who oddly were all for an early de facto exit last September...