You are definitely right in the sense that not many people earn a gross of £100k.
Which after tax, is about £5.5k a month. £66k take home. No child benefit. No childcare fee help. Have to pay my own housing out of that time. Let's call it £800 a month rent and £950 a month for childcare. That's £3.7k after tax, rent and childcare. All of which I've had to earn, with the hours and pressure that comes with being paid a £100k salary.
If I worked 3 days, on a salary of £19k a year, that's a bit over £1.4k I actually earn. And get to do whatever I choose for 2 working days a week. There's my 2 children (I have 3, but will use 2 for this example) so £190 a month CB. With the same rent and childcare as above, I'll get UC top up of £2.1k per month. (Actually £2.7k but they take £600 because of my earnings) So now my "take home" is £3.7k a month, of which I've earned 38% and have almost half the week off. Once I've paid over the rent and childcare that's £1.9k to me.
To suggest that working little over half a week, on under £20k compared to a someone doing more than full time hours on £100k, should result in a disparity of just £1.8k a month take home, hardly delivers an incentive to work. That's a net difference of £21k a year. Between someone on £100k, and someone on £19k working half a week.
£21k difference in take home between someone working full time hours (++ ) earning £100k, and someone with half every week off, earning £19k.