There's almost 5 million self employed people in the UK (ONS 2016 stat). And while there's nothing inherently bad about being self employed, it can hide significant numbers of the underemployed from official stats. There are also almost a million on zero hours contracts (ONS 2017 stat), and any minimum wage gig with consistent hours is a better option where they find it. When you add the underemployed and the unemployed, there's a lot more competition for work than at first glance, albeit varying city from city.
Plus the many more with part-time jobs, often several of them, who spend half their time travelling unpaid from one to another so their hourly rate in terms of cash in the pocket for hours working/waiting between/travelling is much less than minimum. Any job offering more hours will be seized upon.
I'd expect an employer of minimum wage staff to know these things to be honest.