While the data contained in the electoral registers is limited, and much of it is already in the public domain What is not generally in the public domain is the National Insurance number, mandatory on electoral registration since 2015, very useful for impersonation purposes. I wonder if they got those?
While officially they serve a similar purpose, NI numbers aren't used like SSNs are for identification, so the impersonation impact is more limited than an equivalent leak of US SSNs would be.
HMG
https://www.gov.uk/national-insurance/your-national-insuranc...