It is more likely that the UK never benefited because it does not export enough comparatively as I have said. The UK is an importing mercantilist nation that basically funds smaller exporting EU nations by buying their goods.
Tell me again how the UK is going to throw its weight around while representing 13% of EU trade. It is only the fifth or sixth largest exporter in Europe. Germany, France, Italy and The Netherlands represent much greater interests in the EU.
Maybe it never benefited because it did not have enough weight to throw around. This is a blessing in disguise. It is telling the UK that they should seek other trade partners.
The UK should expand its already sizeable trade with the US. No more EU restrictions. Trade with Japan, China, Hong Kong, Canada and the US more freely.