"...it seemed rather too important to stay out of at a time when the UK on its own was struggling."
Yes and now it seems rather too important to stay in, Jim, because the EU is now struggling (among many other reasons, I hasten to add). You shouldn't be surprised that few people in 1975 believed it would morph into a political union, Jim. They were specifically told that it would not and people then, even older people, were more gullible. Some of them actually believed what politicians told them. It was a different world.