The issue here is home ownership vs renting. Magaret Thatcher's government made the prospect of owning your own home achievable for many, many people who would otherwise be renters. That changed the housing market, with the expectation of the next generation that they should own their own homes.
The trouble with home ownership is that you not only have to be able to afford the mortgage, but also the home maintenance, buildings insurance, etc.
Your friends are renting. That means that if their income is below that deemed necessary by the Government, they will get some help towards their rent. The housing benefit allowances are:
£112.55 for the couple
£65.62 per child
So that's a total of £309.41 per week.
Then the following is disregarded from earnings:
Couple: £10
Then the premiums:
Family element £17.40
30 hours element: £17.10
So, they can have an income of £353.91 per week, after deductions, that's £18403.32 per year, or £22920 before tax salary.
The £18403.32 (after deductions) figure is for all income, including tax credits.
Every £ (after deductions) they get after that, will have a deduction of 65p.
In terms of tax credits, they will get an element per child, plus an element for working 30 hrs pls, and a family element. Then, every £ before tax that they get, over £6420, will see 41p deducted from that figure.
If you give a rough idea of salary, you'll be able to very easily work out the likely figures.
The point is that the deductions are fixed and uniform. It will always be better paid to be in work.