As far as I understand from a 'justifiable' point of view, the HSE regard live working as acceptable only in the absence of any realistic alternative.
So if you are live working on a fuseboard that is fitted with multiple means of safe isolation, but you don't want to use them 'to avoid pissing off the client' and work live, this would not sit well.
For suppliers who have no alternative but to cut into live cables, then of course there isn't a practical alternative.