The Sedbergh wasn't that bad. It was a gentle old lady of a slope soarer (something the ACO only did at Halesland). The minimum sink rate was not bad, and it all happened at 32 knots, or maybe a little slower solo. That meant you could wrap it tight into a thermal and outclimb much higher performance gliders. The downside was that between thermals you couldn't increase speed much without the glide angle going into single figures. The Kirby Cadet Mk3 was just an aerial toboggan with very few redeeming features beyond being able to (just) fly a circuit from the meagre height it got from the launch. No chance of a cadet wandering off anywhere else, a "string fling" to 600 feet, four turns and a landing.
As for the debate raging above about blue wings, they were awarded for "completing the course", which generally meant the cadet had already exceeded the allocation of flying and was unlikely to get to solo standard in the foreseeable future.
If a cadet was up to solo standard, but prevented from going solo by, for instance a medical issue, they could be given a "Ghost Solo" where there was a senior instructor in the aircraft, but taking no part in the operation and not speaking. This would lead to the award of silver wings.