"Whether an individual voted in 2015 (General) and 2016 (Local) Elections"
Wouldn't that be against the representation of the peoples acts?
Certainly when I used to work elections, at the end of the vote the electoral registers were sealed up and transported to the count, with the boxes and then after the count secured with the voting slips to be placed in secure storage for many years. Access to any of that would require a High Court Order.
Or are they saying that they have the party's tallies from the people at the exit of the polling stations?
If it is the 2nd, many won't give that data, and postal voters just aren't in it.
And my Twitter account isn't in my name and uses an email address not associated with me in any other way.