Yes and we've been floating between 4th-7th for the last 100 years, inside and outside of the EU, the point you are missing.
You are making the claim that we are only the 5th largest economy now because of our EU membership, but the data appears to show we are 5th regardless of our membership.
It being "a different world" is irrelevant as everyone at all times played by the rules of the day.
For your assertion that only the EU has kept us around 5th to be true, you would have to assume that pre-EU we were consistently batting above our average and that we would have fallen massively in the last 30 years to our natural position of 10th or something.
No, I am going to agree with you actually - we weren't batting above our average - we had the ability to operate in that way and the global economy was such that we were able to maintain that position - at that time. The factors were such that they favoured our abilities and the economy of the time.
I believe the UK no longer has the necessary industries and skills to maintain that position outside of the EU based on export and import data. I'm not saying we don't have relevant industries and skills, I'm saying that what we do have is so heavily interlinked with the rest of Europe that we would be disadvantaging ourselves (and others) because of the mutual reliance that has been formed over the years. For whatever reasons - be it the UK government's decisions in years gone by regarding industry or from the EU modes of operation, I think that at present , we are maintaining our strong position because of the way we operate within the EU. That's how the world is now.
I think you are seeing the country changing from from being out and then in the EU coupled with a relatively consistent economic position and taken that to mean that regardless of what we do, we will be fine. I don't agree with that - I think a decision to drastically change our position in the global economy based on our strong position as part of the EU is a decision based on information out of context.