My only experience on this route is flying SAA. I was in Y and the key bonus was the upper deck of the plane is used as seating for economy but it's a small cabin and the seats have a lot more legroom and space (more like premium economy on other airlines).

Having said that, my flights were nearly two years ago and so things may be different now. I had no problem getting seats on the upper deck at check-in (and rather surprisingly, on the LHR-CPT leg the upper deck was half empty).