The option to donate the political "levy" is tied to the Labour party, as you well know. They were the foundation of the party. If a union member wishes to support another party he is free to opt out and make a donation to whichever party he chooses.
The statistics quoted are only based on a survey. their validity is clearly open to question. The electoral vote is secret and one cannot know how people voted. One can ask a few but there is no guarantee that those questioned revealed the truth.
Public funding of political parties has great advantages. Firstly the amount spent on an election campaign can be constrained by limiting the public funding and banning large donations from companies or individuals. Secondly, the power to influence government policy by means of large donations would be removed, resulting in better decision-making on behalf of the population at large. Thirdly, the cost to the individual would be minimal, amounting to less than 50p per year.