Power to release prisoners on compassionate groundsI can't believe that there isn't more to this (a definition of "compassionate grounds"? a collection of factors to be considered in making the decision?), but that statute does confirm that the decision to release el-Megrahi was discretionary, not mandatory.
(1) The Secretary of State may at any time, if satisfied that there are compassionate grounds justifying the release of a person serving a sentence of imprisonment, release him on licence.
(2) Before so releasing any long-term prisoner or any life prisoner, the Secretary of State shall consult the Parole Board unless the circumstances are such as to render consultation impracticable.
(3) The release of a person under subsection (1) above shall not constitute release for the purpose of a supervised release order.
So, it seems like what we know is that:
(1) The decision to release el-Megrahi was discretionary, not mandatory, and as such was (probably - I'm extrapolating from American law) subject to limited review, if any; and
(2) Based on the history of compassionate release in Scotland, if the prisoner meets the threshold finding of being terminally ill, he gets released.
Given all that, MacAskill treated el-Megrahi like any other prisoner, an amazing bit of equality that I'm not sure he deserved. It also reinforces my perception that MacAskill has balls the size of church bells for following through on it.