Labour neither opposed Brexit nor advocated for a second referendum during the 2019 GE campaign though. We cannot be sure what their voters' position on Brexit was. My assumption is that the majority of them were opposed, but a significant minority were still in favour, and we do not know how big that significant minority was, we do not know whether it would have been enough for the outcome of a new referendum to be different from the previous one.

> Labour neither opposed Brexit nor advocated for a second referendum during the 2019 GE campaign though.

Yes they did. From their party manifesto[1]:

> Within three months of coming to power, a Labour government will secure a sensible deal. And within six months, we will put that deal to a public vote alongside the option to remain.

Large numbers of voters switched from Labour to Conservative because of Brexit. It's probably fair to count all Labour voters from that election as "not in favour of Brexit".

1. https://labour.org.uk/wp-content/uploads/2019/11/Real-Change...

That "sensible deal" is still Brexit, it's just not a no-deal Brexit or hard Brexit. I had missed the bit about putting that new different deal to a public vote though.