What does "Muddy Wellies" mean in British Idiom?

I subscribe to The Economist, a British publication. Normally, the writing is clear. Every now and then, the articles contain an obscure British idiom. In the last issue, an article referred to David Cameron as a Tory "of the muddy wellies variety." I have no idea what that means. I ume "wellies" refers to Wellington boots. I ume the reference is to muddy boots. But I still have no idea what it means. What is a "muddy wellies" person?