Execution of mentally impaired may be subject to the prerogative of mercy
Pitman & Anor v The State (Trinidad and Tobago)  UKPC 6
Both appellants, inter alia, contended that a death sentence was unlawful for an individual suffering from their level of mental impairment. However, both individuals had already had their death sentence substituted for life imprisonment. The appeals were dismissed.
Alison MacDonald QC was involved in this case.
JCPC Press Summaryhttps://www.jcpc.uk/cases/docs/jcpc-2014-0084-press-summary.pdf