The way I see UBI working is that Job Seekers Allowance (the current name for unemployment benefit), or its equivalent Universal Credit, simply has the requirement to look for work removed.
It is currently £84.80 per week for over-25s. Just over £4,400 per year or £367 per month.
If you are unemployed, you would not need to look for work. All those people whose job it is to harass /cajole you into work are now redundant.
If you are already on any pension or benefit which pays more than this rate, you don't get any extra - your current pension / benefit is taken to include the UBI.
Housing benefit and council tax benefit are totally separate and continue as before (pending massive, massive council house building programme and total overhaul of housing - which is a separate issue to UBI)
For people in work, the tax system is adjusted, so that someone doing 37.5 hours per week on adult minimum wage (or more) is no better off - the UBI is paid through PAYE, but the tax free allowance and/or the tax rate adjusted to give the same net pay. (The level of NMW is also a separate issue).
Everyone who is working is paid UBI through the PAYE system, or for the self employed via their tax return. For people on very low wages and low hours, this would be a net extra added to their pay.
The other extra costs would be that it is now payable to every adult over 18 years. So anyone who currently doesn't work or claim benefits or pension - e.g. all SAHP. This extra cost would be met through general taxation, preferably on assets of the wealthiest, not necessarily on income.
Taking away the work requirement means that if you are happy to live on £84.80 per week, you are welcome to do so, at the taxpayers expense. It would be enough to buy food and second-hand clothes, but you would have to sofa-surf / live in a van / live with someone else who pays the bills, etc.
The advantages of putting such a UBI system in place is that it would ease the handling of a transition to a post-AI world, would put cash directly in the hands of SAHP who in cases of financial abuse may not have any other access to money, and it would de-link survival-level income from work.
We need to abandon the idea that everyone should work for their food.