Earlier quoted context omitted.
"Let's agree that these are not the rich people we want to tax." 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)
The average taxpayer with income between £20k and £30k pays on average £2630 tax, some 10.7%. If you hand him over £15k and them tax him on average additional £15k then his tax rate must be around 44%. This will be extremely demotivational. A hard-working person will have barely more than absolute freeloader and if he tries to work extra hard you will take half of his reward.
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?