I contacted the Cetacean Strandings Investigation Programme, but they didn't really have much clue based on description or pictures.
They suggest it was a harbour porpoise (apparently they make a nearly daily sacrifice to keep the land walkers appeased, at 300 UK strandings per year) but remarked it was looking thin.
CSIP postulated the brain was eaten out by scavengers.
I tentatively imagine the lil shamu may have
Brain Bite itself one time too many times and crossed the thin red line between life and death.