After a normal circuit the aircraft crossed the threshold at 85 knots. The pilot advised that the wheels locked as soon as braking was applied. Intermittent brake application had little effect in slowing the aircraft and as the pilot considered that insufficient strip remained to permit a go-around he attempted to ground loop the aircraft. It slid off the side of the strip and collided with a fence before coming to rest. The strip surface was very slippery as a result of overnight rain. Misty rain was still falling as the pilot made his approach and touched down about one quarter of the way along the strip. The pilot had not determined the last point at which a go-around could be commenced and persevered with the landing attempt beyond the point where a go-around could have been safely accomplished.