The government runs two huge databases. One based on driving licence/passport information, the other on national insurance numbers. Access to government services is moving to cross-checking these databases to limit fraud possibilities. Voter registration by councils has access to the work and pensions database (based around NI numbers) to check on would-be voters but then distills this information on to the voter's registration database, which can be searched by an endless array of dodgy companies and by individuals if they pay a small fee to these companies.
There is a strong possibility that such a movement of individual data is illegal under EU data protection laws - certainly, if you refuse to give your NI number to the council they will back off if you mention such protection rights. Usually, the council just cross checks the info they have on the voter with the database, without informing the voter nor giving him or her the choice of whether they want their NI number and DOB on the voter's register (whether the public search box is ticked or not it makes no difference to the access of information on the database as far as the companies that access it are concerned).
Of course, MP's and serious VIPs have the option to register via the anonymous register which can only be searched by the likes of the police and tax man. Joe Public has to get a court order to be included on this register, along with dire warnings that things like mortgages will become impossible as banks, etc., won't be able to access this information.
Then, a database exists that protects individuals' information but they aren't allowed on it because the government deems that information not important enough to protect! Arguably, as well as a breach of EU data protection laws it is a breach of European human rights.
The idea that one branch of the UK government will protect individual rights against another branch of that government is, at best, optimistic, especially when compared with the fear with which UK govn workers exhibit when you threaten them with being reported to various EU bodies. Post Brexit, then, the ability of individuals to protect their data needs to be urgently upgraded so that they have the basic right to stop non-government bodies from accessing it!