But they are not, right now. The problem of starving people dying in the ditches by the side of the road has been solved without the UBI.

These problems have been 'solved' via a combination of welfare benefits (broadly understood; including food stamps, subsidized housing etc.) and heavy-handed labor market regulation which in practice leaves the most vulnerable unable to get a job at all, and in danger of losing their very freedom as they turn to crime in response. In other words, this is so costly a 'solution' to the issue that it's barely a solution at all. UBI would be radically simpler and more effective.

From the point of "enough calories to not starve and enough warmth to not freeze" you have a continuous scale going up and I don't know on which basis will you decide that some point on this scale is "reasonably objective".

True. I think the closest thing an 'objective' answer is that it's not just about calories/food and shelter, but the ability to form something like a community, and pull yourselves up by your own bootstraps from that point. (The Amish, Memnonites etc are another interesting example here.) Living in a concentration camp won't cut it, true, but you don't need that much more.