I definitely do not agree. BI is only viable if the average taxpayer sees no or very little net benefit from BI. So the average taxpayer would pay £15k extra in taxes if they receive £15k in BI. And £42k is above average (but not a lot, the medium income is £21k, but only a little more than half of adults are taxpayers)
More importantly, if both him & the freeloader are spending £15k on rent & food, then the freeloader has £0k of disposable income and the worker has infinitely more disposable income.
Another way of looking at it is that average taxpayer with UBI has an income of between £35k and £45k and has a marginal rate not much different from what people making £45k are paying right now. People making £45k aren't currently significantly demotivated by taxes, are they?
"the average taxpayer would pay £15k extra in taxes if they receive £15k in BI"
vs
"Another way of looking at it is that average taxpayer with UBI has an income of between £35k and £45k".
Today a taxpayer with median income £21k takes home £19k net. After your reform, he will take home the same £19k net (remember: pay £15k extra in taxes if they receive £15k in BI). Except hew will now be way below the median and making only £4k more than the now ubiquitous dude Lebowski type.
There are about 31 million taxpayers in the UK. The basic income tax rate of 20% for income up to £42k/year generates 33% of the income tax revenue. Let's agree that these are not the rich people we want to tax. (88,7% of Britons earn less than £50k/year.)
The higher (40%) and additional (45%) tax rates generate the rest 67% or £117 billion of the £174 billion income tax revenue.
In order to raise additional £472 billion, we would have to increase the higher and additional tax rates to approximately 211%.
As you can see, taxing the "rich" cannot raise enough money to give everyone unconditional poverty line minimum income.
Source: https://www.gov.uk/government/statistics/income-tax-liabilit..., https://www.gov.uk/income-tax-rates