The POD would be to somehow do away with Jacques Delors' tenure as head of the European Commission.
Delors was a socialist, as well as a major politician from a major country. Some EC heads have fit the profile of "major politician from a major country" but probably more have been minor country pols, like the current one. The former tend to be stronger leaders of the Commission and Delors fit that mold. He did more to push European integration than the norm, and also reached out to the left, which had tended to oppose European institutions, by including strong worker protections in the various integrationist treaties.
The British left, particularly in Labour, starting seeing European institutions as a counterweight to Thatcherism, and their leadership still sees things that way, judging by the pressure on Corbyn, who is obviously eurosceptic, to run a stronger pro-EU campaign. Though the nationalist right might have opposed Britain being part of a pro-integrated Europe anyway, the additional worker protections swung some hyper-capitalist against the EU as well.
Not incidentally, despite their rhetoric, the actual position of the Conservative Party has always been in favor of British membership of the EU or its predecessor organizations, but with the ability to opt out of specific policies that they don't like. Labour's position has changed more, from being against British membership or at least divided, to being in favor, to being divided or very weakly in favor.