I don't know about the UK, but in the US, you effectively have a choice between getting welfare and working. So people have the option to take some low-paying job, but they don't do it because then they lose welfare benefits that pay almost as much, sometimes subsidizing childcare etc. so it's an unemployment trap. UBI is much saner because you get it in addition to any income you earn from working, so you can go ahead and take that low-paying job as well.