and a leftist argument could be make that it will allow better protections for workers or vulnerable groups that the EU would never accept
I have no idea where this myth is coming from. Have you ever tried to sack someone in Germany? DH just spent 6 months in a tight negotiation with a guy who had to be sacked because his work was terrible but despite 12 months worth of documented below par performance (and other issues), worker protection laws are so stringent in Germany that employers can end up paying a fortune (as happened in this case). DH's employer is a US company and I did urge him to inform them before opening a branch in Germany that you couldn't just hire and fire like they do in the States, especially not women, but they honestly couldn't imagine how strongly protected employees are there.
Same goes for mothers and pregnant women - they have the automatic right to paid leave for the last six weeks before and the first eight weeks after the birth (the former is voluntary, the latter is mandatory leave). At an absolute minimum, they are also protected from being fired during and for four months after pregnancy, even if they miscarry.
Each parent has the right to 3 years unpaid leave until the child is eight, and on top of a generous child benefit paid by the state which increases with the second and third child, they also receive what is called "Elterngeld", an income replacement benefit that ranges from €300 to €1800 for those who reduce their hours to between 30hrs and 0 hrs per week for up to 12 months (this includes self-employed parents, freelancers and those who aren't working).
And so on. There may be a veritable jungle of rules in Germany, they do love their rules, but these rules protect workers, pregnant women, parents etc way way beyond anything the UK does and Germany is still in the EU.
It's not perfect, by no means is it anywhere near perfect, but it's a damn sight better than what successive UK government have provided. Of course, the reasons for that lie in the fact that unions are strong, union membership is high and providing social security is very much a conservative concern.
So the German Conservatives don't believe in "big society" or in a small state or anything like that. They subscribe to the idea that a happy workforce is more productive, hence much better conditions including for childcare and its costs. (My mother whose charity provides all kind of services - counselling, childcare, personal care, women's refuge etc) was certainly cured from voting Labour when the German Social Democrat Government before Merkel wholeheartedly embraced Neoliberalism and cut cut cut.)
The problem about getting in cheap workers has actually also been identified by the EU and rectified via a new law, but if you're only listening to Corbyn on this, you're bound to be unaware because he seems to not know any of this.
And btw, these worker protections don't just exist in Germany or Scandinavian and Western European countries, there are a number of excellent provisions also in Eastern European member states (and as we know there are of course shitty rules in others).
Fact is, UK workers get a rough deal in many respects, but that's not the fault of the EU and I find it naive in the extreme to think that leaving the EU is going to lead to better conditions.