I already explained it. You let him go home to die, on the basis that he's likely to win his appeal, leaving you with egg on your face, nobody guilty of the crime, and the real killer still free (incidentally he still is.) Instead of that, he gets to go home right now, but only on condition that he withdraws his appeal. He agrees, because by the time the appeal is heard, and he wins, he might die in prison in Scotland.
From the Scottish Criminal Cases Review Board...
" ...27.216 In accordance with the principles set out at the beginning of this chapter the
Commission has also considered whether, notwithstanding its conclusion that a
miscarriage of justice may have occurred, the entirety of the evidence considered by it
points irrefutably to the applicant’s guilt. The Commission’s conclusion is that it does
not.
27.217 In these circumstances the Commission believes not only that there may have
been a miscarriage of justice in the applicant’s case, but also that it is in the interests
of justice to refer the case to the High Court. The Commission accordingly does so. "
http://login.heraldscotland.com/SCCRC-Statement-of-Reasons-red.pdf