By my reckoning you could afford to pay every adult in the UK £10,000 a year out of current tax receipts, provided you abolished all other forms of government spending. So no NHS, no schools, no road maintenance, no army, no civil service. No-one to collect the tax in the first place.
If you wanted to keep those other things too you'd need to double taxation. Let's say 50% on all earned income plus the same in corporation tax. Businesses relocate abroad, jobs become scarcer, and of course there's a growing sense of resentment between those with jobs and those without, with one lot envying the greater spending power and the other lot sneering at the scroungers and slackers whose lifestyles are being funded by their hard work.
Then of course inflation goes up, because there's all this unearned money sloshing around. Suddenly £10,000 is enough to live on for six months, not a year. They propose putting up taxes again to pay for it and the brain drain of high earners begins in earnest.
After a couple of generations the only people left are those too idle or vulnerable to hold down jobs and the tax take implodes. Under pressure from the IMF the citizens' income is abolished. There are riots and extremist parties start to gain popularity.
In short it's a stupid idea that would result in disaster. The Greens come up with this sort of nonsense policy because they know they'll never be elected.