My wife had half a tonne of angry venison charge into the side of her car a few years back and the bumpers remained completely undamaged. The driver's door and o/s front wing were completely f**ked though, as was Bambi's Dad, which brained itself and died at the scene.
What you ideally need here in Norfolk is the kind of anti-RPG cage fitted to armoured vehicles in Afghanistan -
Oh, and bumpers look cool on some cars - Dino/356/XKSS - but not others - MGB (rubbers).