My hope is that Labour are playing this smart. They'll bang on about how Brexit won't change, but that "we'll look to increase economic and social strengths via our relationship with the EU". We'll reintroduce entry to the single market, ensure freedom of movement, and basically rejoin in everything but name - and then eventually say "well, if we want to rejoin it's basically a tick in a box".
The EU will likely be happy for the UK to rejoin, even without punishment. The most reliable ally in the battle against Euroscepticism is a former Eurosceptic that can say how shit things were after leaving, and how much better they are since rejoining.