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with #GDPR how do I ask them to delete them?
As others have said, it's not a GDPR issue, it's a fundamental problem with the way the electoral roll is treated. Letting candidates use it for electoral communications is the least of the problem, although abuse of the list by political parties could be eliminated if they didn't get given the list in the first place but just sent their one page to the returning officer and had him forward it it to each registered elector.
we have a by-election and I discovered almost all parties have my details, with #GDPR how do I ask them to delete them?