First, they do not allow for clawing the basic income back in tax from higher-income earners, which could be done with no net cost to the affluent or to the Exchequer, simply by tweaking tax rates and allowances so that the extra tax take equals the basic income paid.

Second, they do not take account of administrative savings from removal of means testing and behaviour conditions. Administration accounted for £8 billion of the £172 billion 2013-14 budget of the UK's department of Work and Pensions, much of which will have gone to pay staff in local job centres to monitor and sanction benefit recipients. This does not include hundreds of millions of pounds paid to private contractors to carry out so-called 'work assessment' tests on people with disabilities, which have led to denial of benefits to some of society's most vulnerable people.

Third, they compare the cost of a basic income with the existing welfare budget and assume that all other areas of public spending remain intact. Yet governments can always choose to realign spending priorities. The UK government could save billions by scrapping the plan to replace the Trident nuclear missile system, now estimated to cost more than £200 billion over its lifetime. It could save further billions by ending subsidies that go predominantly to corporations and the affluent.

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Fourth, back-of-the-envelope exercises ignore the wide array of tax exemptions and allowances that have come to characterize the modern fiscal system. The UK personal income tax allowance, which in 2016 exempted the first £11,000 of income from tax, costs the Exchequer almost £100 billion a year in foregone revenue. Non-imposition of national insurance on some earners costs another £50 billion. These two exemptions alone amount to nearly 10 per cent of GDP and are strongly regressive.