If you don't produce it directly or demand it as taxes, you can't guarantee that it gets produced at all. And you need to guarantee that, if you're going to go around giving it away for free.
You don't need a bureaucracy to figure anything out. I have already done it. Everyone gets 2 L of bottled drinking water, 2200 kcal of food, a lockable room about 8'x10'x8.5', tap water, electricity, and Internet. The utility numbers mostly came from current usage statistics, the room size is based on solitary-confinement prison cells and self-storage units, and food and water based on the generally recognized dietary subsistence level of 2000 kcal/day and 1 mL/kcal rule of thumb for drinking water. An easy way to do it is to rely on the Geneva Conventions. If you cannot supply conditions sufficient for humane treatment of refugees and prisoners of war to your own citizens and residents, your numbers are wrong.
If you don't like the food you get with your ration, sell it. I'm sure there will be a service around to collect your unwanted UBI tangible goods and resell them to someone else in exchange for a cut of the proceeds, so you can get more, different, or better of something else that you do want. If you're a petite lady, you could likely trade in your 2200 kcal of basic UBI food for enough cash to buy 1800 kcal of better food. If you don't use all your free electricity, you could use it to mine bitcoins or purify copper electrode or recharge batteries. If you don't use your tap water, you could do someone else's laundry for money. But I picked the numbers to be in the "this kinda sucks" territory for a typical American. You'll probably always want more than you get, and for that, you still need to access the paid trade markets. And if for some reason you don't already want more than you get, that's what advertising is for.
The same calculation problem exists with the cash-only vision of UBI. Someone has to decide how much is enough. But then that number constantly changes as prices adjust to what people actually buy with their "enough".
The point of the tax thing is that the US has long been conditioned to be averse to communism in any recognizable form, and a less sizeable fraction is also averse to fascism. The fact remains that whatever you want to give away for free has to be produced first, and you can't guarantee that a for-profit business would make enough as long as it is possible to earn higher profit by producing less. If the government needs 750 Tcal of food to meet its UBI requirements, it can do one of 3 things: own farming capacity sufficient to produce it, order existing agribusiness to produce at least that amount and surrender it, or tax/print whatever it takes to buy that much on the open market. If communism is unacceptable, you have to do one of the other things, and the latter has the potential to destabilize your entire monetary system.