The threshold for income tax should be the median wage for full-time work (currently about £23,000 per annum), with tax thereafter at a flat rate with no further exemptions or allowances. That this would mean the same rate for everyone does not, of course, mean that it would be the same sum for everyone.
It does, however, mean the unaccustomed arrival of a tax bill for those whose only current relationship to the taxation system is that of being bailed out by taxpayers when they look like becoming so poor that they might join our number.
There are many uses to which the vast revenues thus accessed for the first time might be put, not including, for example, nuclear weapons, or wars in Afghanistan and Iraq, or bailing out super-rich non-taxpayers in order to keep them super-rich and non-taxpaying.
One such good use would and should be the amalgamation of all current Social Security payments into something called, and providing, Social Security, and guaranteeing that no one's income ever fell below half the national median wage for full-time work.
Both this new tax system and this new benefit system would be dazzling simple to understand, and would cost next to nothing to administer.