The political consensus throughout the 1960s and the early 1970s
- whether you agree or not with those "experts" -
was that the Uk had missed the boat when it didn't oarticipate with the original 6 countries in forming the then EEC
Trade with nearest neighbours is usually the most important trade and the UK had too little influence in how the rules for European trade were being formed.
That's why the UK joined
Not because if feeling "European" but because the Uk trade experts, economists and most politicians thought the UK was missing out.
Even if you disagree with the decision to join...
Leaving is totally different to never having joined
Brexit means Year Zero - the UK in modern times has never been without any trade deals or lacking vital agency & organizational memberships
The UK had all those trade connections & memberships before joining the EEC
Now it only has these as part of the EU and will lose them on Brexit
The UK had an egg, chose to be part of an omelette, now wants back its egg
It will take years to get back to that pre-EU state of connectiveness in trade, agencies, organizational memberships
Of course the Uk will - eventually - negotiate a trade deal after Brexit
BUT
It won't remotely resemble the frictionless trade and movement of goods & services we have now.
Trade will be hammered
and it will take YEARS to renegotiate all the trade deals with the rest of the world
What Canada gets from CETA is nothing like what the UK needs from its nearest neighbours.
What Turkey has is nothing like the UK needs
Even the US with its 50 or so MRAs with the EU does not have the frictionless trade that the UK needs