There are lots of simple things that could be done to improve the funding of political parties. But a politics tax isn’t one of them.
A cap on donations from a person, company or organisation would be a good start, the Conservatives have in the past been too reliant on a small number of donors, and this makes us inward rather than outward looking. The Labour party is now almost totally funded by the unions who excerpt an inappropriate level of influence over it. And the Lib Dems have had their own troubles.
It would be better if political parties genuinely looked to have a broad base of financial support. Modern digital media makes it easier to get small donations from ten or even hundreds of thousands of people and doing so would force all parties to listen to their own grass roots.
State funding of political parties would be hugely unpopular and morally wrong.