Why do you assume that doing badly= coming from a poor background and also doing well= coming from a rich background? Have you considered a poor person succeeding and rising up to the top through exam success?
This approach means less people get top marks and so genuinely good pupils do well, rich or poor, universities will be able to distinguish the good from the bad.
Under labour's equality system everyone does well so everyone stays poor and no one can make it to the top. Equality does not equal fair, look at the Soviet Union. They were all poor, but all equal so it's okay!