I'm not convinced it's a viable option, other than for young people living at home whose parents are happy for them not to contribute anything towards food etc and subsidise them for other stuff, too.
The UC rate for under-25s works out a few pence over £73 a week, about the same as they would get for just one day's work at NMW for an 18 year-old. If they worked just 2 days pw, they'd double their income.
All the young people I know got p/t jobs as soon as they were old enough, with the exception of one who is ND and has a raft of health issues. One was working 2 evenings a week as a kitchen porter at 15, which I'm not sure is even legal. They all wanted their own money, and driving lessons as soon as they were old enough. I'm in the SE though, and I recognise that there may not be the same employment opportunities in other regions. They mostly worked in retail or hospitality.
If the problem is companies not wanting to take on school leavers, perhaps they should be incentivised to do so.