I predict global slavery to emerge sooner than UBI will become a reality. Call me a pessimist if you wish

There's no reason Georgism requires a global mandate. Any level of government that levies taxes can replace their current taxes with land/resource/pollution taxes.

In most countries the national government levies the most tax (usually as income and company taxes), and pays out the most in benefits, so they are the level that needs to change most. There are no global taxes that need to be replaced, just local, state, and national.

UBI didn't work for the Communists, can't see how it's going to happen in a fractious democracy.

I think a the biggest "global democratic mandate" ever seen is functionally a dictatorship... And apart from the Communists, dictatorship don't have the sort of priorities that lead to UBI.