Well the vegan society wants to offer money to diary farmers to help them switch to plant based farming.

That probably is a good idea. Remove subsidies that prop up failing industries like dairy and use then instead to encourage farmers to switch to plant based farming. It will take a lot of investment to transform a dairy farm into something capable of producing crops, so it's almost certain government help will be needed there.

Finally I don't want to leave the EU and I'm hoping it comes to pass that we stay in after all. But then our agricultural policy is decided at EU level to a large extent.

Then again the EU is more likely to act on climate change and be sympathetic to left wing vegan notions than the UK govt.