You're forgetting about the hundreds of random benefits that could be abolished if UBI is brought in.
Adult social care, attendance allowance, bereavement allowance, bereavement benefit, carer's allowance, child benefit, cold weather payment, community care grant, council tax benefit, crisis loans, disability living allowance, employment and support allowance, funeral payments, guardian's allowance, healthy start scheme, housing benefit, in work credit, incapacity benefit, income support, invalidity benefit, job grant, jobseeker's allowance, local housing allowance, maternity allowance, mobility support, over 80's pension, pension credit, reduced earnings allowance, retirement allowance, return to work credit, school uniform allowances, statutory adoption pay, statutory maternity/paternity pay, statutory sick pay, sure start maternity grant, tax credits, training premium, travel to interview scheme, unemployability supplement, war disablement pension, war widow's pension, winter fuel payment, working tax credit...
And the tens of billions of pounds it costs to employ paper pushers and keyboard mashers to apply for, assess, grant, check up on, pay, cancel, reassess, and deal with problems with said benefits.
Get rid of EVERYTHING. Pay a UBI high enough to live no matter what your circumstances. Ignore the wails of protest from everyone currently in receipt of those benefits. Yeah, you're going to piss off the war widows with disabled carers that are losing their extra £100 a month or whatever. And the "I worked all my life for my savings so why should everyone else now be getting free money????" outrageds. Tough.
And make the corporations pay their fucking corporation tax on business activities in the UK.
Then the figures add up, and it boils down to the will to try something radical.