Now, imagine that budget negotiations are always done in percentages of the total (balanced) budget, such that the state spends exactly 8% of last year's GDP.
Let this framework be written into the constitution. That is, imagine a government that does not have the power to fiddle with your tax rate or your welfare benefits. Its only job would be to prioritize among the various services the state provides to its citizens. Imagine a state that has no interest in your personal income or business revenue, but pays you 25% of per capita GDP in monthly installments.
To get from here to there, imagine a twenty-year phase-in period, during which the welfare burden is gradually shifted to the UBI and the tax burden is gradually shifted to the LVT. This could be done under current constitutional authority. Sometime during those twenty years, however, the constitution would be amended to forbid the making of laws that collect taxes or distribute welfare on a means-tested basis.
Finally, imagine a political party that abstained from all voting in your parliament that did not further this agenda. Imagine a party that had no interest in starting or ending wars, progressive or conservative education, supporting the arts or funding the sciences. These things it would leave to the other parties to negotiate until Utopia has its constitution.