Trouble is that you'd need to have hefty tax rates, no "nil" rate lower thresholds, etc to make up the increased cost.
I.e. someone currently earning £25k pays a few thousand in tax/NIC. If you "give" them £25k, they now "cost" the country £25k PLUS the few thousand in lost tax revenue, so they may "cost" the country say £29k. So if they carried on working to earn £25k gross, even with a 50% tax/nic rate, they'd only contribute £12.5k back to the treasury, meaning The treasury would be out of pocket by £16.5k.
Someone would need to go on an earn £58k with a 50% tax rate, for UK PLC to be back in "break even" mode, i.e. £29k tax on the £58k wage, to cover the £25k basic income plus original tax loss.
You'd need a marginal tax/nic rate of around 75% to start to make a dent in the cost of UBI for those earning more than average. And that's not very behaviourally acceptable. Would people really put themselves out to work, commute, etc if they only "took home" less than 25% of their earnings?
What about the cost of pensioners, currently on state pensions of say £12k per year and who are managing fine, especially a couple, especially if they also have occupational pensions. They don't "need" more money via UBI but would get it anyway, another "cost" to the taxpayer.
Then you have those who couldn't live on UBI. Even £25k isn't enough in expensive areas to rent a house, and what about disabled who'd need more to cover extra costs of living etc? So you couldn't scrap the benefits system - you'd still need "top ups" for those struggling on UBI!
The fact is that UBI would need to be at a high level so that "most" people "could" live on it if they wanted to, i.e. don't need benefits. But the cost of that is extortionate meaning very high tax/NIC rates, which makes working less attractive.
The number crunching really doesn't stack up.