That's not how I remember it. Or indeed what some of the 'pro' leaflets I still have said. All sorts of integration beyond the Common Market was indeed discussed, like currency union, common borders, giving up a UK passport for a EU one, EU courts, EU courts, EU army, EU-level foreign service, embassies, etc. The 'discussion' took the form of categoric, flat-out promises that it would not happen. Oh no, it was ONLY about trade, a common market, etc.
Note: For the pedantic types, in the above, where I refer to EU this or that, I'm using "EU" as a shorthand for a common-market wide provision, not the EU as we know it which, of course, didn't exist in those days.
Heath even admitted years later, Zurich about 1990 IIRC, that that had been a lie, and that when he took us in to the Common Market in the 70's, the actual objective, even then, was a Federal European superstate a la Jacques de Lors, etc. They just knew they couldn't call it that or we'd never stand for it.