You don't get. Fighting to stay in the EU means fighting to be forever bound by rules that make it impossible for a Labour government to to implement any policies with Labour values. If nationalization is all-but-impossible, budgets must always be balanced and taxes on the wealthy must never be significantly raised, nothing humane or egalitarian or economically democratic can ever be done.No principles, just events
And it is impossible to ever change any of the EU rules on those questions.
European unity and free movement of peoples never had to be coupled with permanent Blairism-Thatcherism.