Technically, yes, it is. There are two questions that would need to be looked at to prove it in this case:
1) is it provable that the cabinet/PM were grossly negligent in their duty of care.
2) The bigger question is was this voluntary or involuntary, i.e. was a decision made to let old people die without giving them proper medical support and a fighting chance (voluntary manslaughter), OR did people unnecessarily die as a result of more benign, but flawed set of decisions (involuntarily). Source:
https://www.lawteacher.net/modules/criminal-law/fatal-offences/manslaughter/lecture.php
These would be difficult to prove either way, however I would argue this governments culpability in 'manslaughter' of our elderly care home residents, by refusing to admit them to hospital (seemingly to avoid the embarrassment of people dying in corridors like Italy), is closer to manslaughter than Blair's Iraq war actions.