Could be a data scam, when it asks for postcodes they could be looking at where there is clusters of higher sympathies towards certain parties, even if you don't declare it that way. For parties like Reform they can know where to push their limited footsoldiers to go from door to door, or direct advertising with limited funds.
I had 23% tendencies towards Reform , if you knock it up to 30 to 40 % and then someone come knocking on my door and starts highlighting concern in areas that I ticked off, you might have another person who votes reform. Even if they don't win the election they can claim a bigger stage in future elections because of a higher % of the vote.