Personally, I think this is a moronic, and almost masochistic, policy. I can certainly understand arguing for sensible regulations on non-EU immigration which is in some cases harmful to poor workers, but this is just going to deter skilled labour from other countries, which ultimately isn't good for society and the economy.
Some will say that unemployed native workers can fill the vacated positions, meaning there won't be negative effects. There are several problems with this - firstly it's a lump of labour fallacy, ignoring the economic benefits of immigration; secondly, it's unlikely that the most skilled positions which could be vacated as a result of this will be filled by people who are currently unemployed; thirdly, small agricultural businesses are going to be culled as a result of this. Many rely on cheap seasonal agricultural labour, particularly from Europe, which British workers don't want to do for a whole host of reasons. They need these workers, yet obviously can't afford to pay thousands of pounds just in order to employ them, so they're left with literally no options. Meanwhile, the stigma around foreigners in the country could grow as a result of this.