Re: Methinks the political spin is strong in this one.
If that data is not withheld when individuals opt out of the public register I would question the legitimate use of my data under GDPR.
In line with the article though, two questions
1) was the non-public register data available to Mr Hacker and
2) were those impacted notified in a reasonable time by the organisation concerned?
Given "40million" I'd suggest answers of yes and, to my knowledge as a voter registered on the electoral roll, no.
Which beggars the question, what's the point of pursuing the case under GDPR as, even after the case is closed, those responsible have done nothing to mitigate the situation for those impacted?
If Google or Amazon or whoever ignored a similar situation there'd be baying for blood from all and sundry but, because it's only Government incompetence, it's ok.