The big issue with UBI entirely replacing the welfare system is that it ignores those who require more money due to disabilities. Person X can subsist on £Y/week, but person Z needs fooprazatine which costs a non-negligable quantity of £Y (there are also other, more nuanced scenarios but drugs come to mind first). Of course there are ways around it and medical benefit fraud is already incredibly low (it's hard to fake that you need a carer or an expensive regimen of drugs (painkillers notwithstanding)).