Nice to see that they've sourced that article wel....oh wait.
If they do introduce compulsory insurance, it will be for farm vehicles and will be at a very different premium to car insurance, plus they will obviously distingusih between commercial sit-own mowers, like the council's (which probably need insurance already as they use roads) or farmers with large areas of land, and those who simply own a mower to make mowing their large garden a bit easier.
I assume from the one useful thing they said in the article that this is to protect farm workers or visitors, so I'm sure they'll have measures to exclude those for whom there is no risk of third party injury and the cost would be disproportionate.