Biggus:
"However, from the "company's" point of view, why shouldn't it retain fit well performing individuals in preference to unfit individuals who aren't performing as well?"
One reason would be to ensure that the overall numbers don't plummet below the requirement. Do you really think that those people who were really keen to leave, but weren't selected, will hang around for very long? I suspect that most of them will be gone 12 months from now and then you'll see shortages develop - just like after the last redundancy round.
A much more realistic approach would have been to select all of the volunteers for redundancy, and then use the reverse appraisal to decide who goes compulsorily. I'm probably wrong, but legally, I thought that was the way things had to happen within each trade/qualification/skill.