There is a lot said about Jeremy Corbyn and I've noticed that a lot of the mainstream media (even left-wing like the Guardian) are very against him and have been vocal about everything he stands for, which I find interesting. This article,
https://www.politico.eu/article/jeremy-corbyn-brexit-referen...
from 2016 says that he thought at that time we should stay in EU and this does align with what I remember of that time. He was widely quoted as saying that although EU needs to change, we should stick with it and help change it from within.