Define unsustainable? I hardly think importing tomatoes from the continent is unsustainable.So are you in favour of unsustainable food lines?
If you feel this strongly about the state of food in the UK, why not start (or join?) a consumer-led movement to convince the public and retailers to buy/sell more responsible produce?
It would be a damn-sight better way of achieving your goals than economic self-flagellation, leading to enforced price-hikes and/or shortages.