A trade/economic community was enough then, and was all that has ever been endorsed by referendum.
Joining EFTA in 1960 meant Britain could retain strong ties with the Commonwealth. However as the 1960s progressed it became apparent that EFTA membership wasn’t enough to stop our economic decline. Commonwealth countries wanted access to Britain’s markets but were less interested in allowing Britain free access to theirs. We actually left EFTA to join the EU, specifically because we felt free trade was not enough. The Wilson government, setting out its reasons for applying in 1967, stressed that Europe is now faced with the opportunity of a great move forward in political unity & we can — & indeed we must — play our full part in it.
hansard.millbanksystems.com/lords/1967/may/02/the-european-economic-community-british
In the 1975 referendum it was quite clear that the that being a member of the EEC was more than just a trade/economic community. Apart from newspaper archives the Government & the “Yes” & “No” campaigns sent leaflets to every household.
The Government’s leaflet said if the UK left the EEC;
Britain would no longer have any say in the future economic & political development of the Common Market
civitas.org.uk/content/files/1975ReferendumGov.pdf
The “In” campaign said;
Why can’t we go it alone?
To some this sounds attractive. Mind our own business. Make our own decisions. Pull up the drawbridge. In the modern world it just is not practicable. It wasn’t even 40 or 60 years ago. The world’s troubles, the world’s wars inevitably dragged us in. Much better to work together to prevent them happening.
Today we are even more dependent on what happens outside. Our trade, our food, our defence cannot be wholly within our control. That is why so much of the argument about sovereignty is a false one…..
... If we came out, the Community would go on taking decisions which affect us vitally - but we should have no say in them.
civitas.org.uk/content/files/1975ReferendumYES.pdf
The “No” leaflet went on in some length about sovereignty;
civitas.org.uk/content/files/1975ReferendumNO.pdf