>>154585English trade unions were the main opposition to joining the EU/EEC, with some unions such as the RMT supporting Brexit in 2016. As for why they switched their position, I don't know the exact reasoning but there was a general shift to pro-europeanism in the British left during the Kinnock and Blair years. I guess as the old guard of trade union leaders died out they were replaced by younger, more liberal types.
Brexit was something that was promoted by the right in the UK, although the far-left broadly supported it. In the Netherlands neither the far-right nor far-left support leaving the EU, so I doubt centrist farmers unions would take that position.