In the UK, if you are out of work, the government gives you cash, jobseekers allowance, about £3.8k a year.
If you in work, you effectively get cash through your personal allowance, at your top tax rate, betweeon 20% and 45%. The jobseekers allowance amount of £3.8k is 33% of the standard personal allowance.
So, rename the jobseekers allowance a UBI, give it to everyone and get rid of the personal alalowance.
Gets a UBI in place, and has no great financial effect. It does, however, save some costs of administering the jobseekers allowance and personal allowances, benefits those in lowly paid employment at the expense of those paying higher tax rates.
It also, if you are on the left, removes the stigma of those unfortunates stuck on benefits or, if you are on the right, means that lazy people sponging off the state have more incentive to go to work: their income is more likely to rise if they get a job.