[Kizzy]

He had been on a disability benefit for the last 2yrs ( which is how contribution based JSA is calculated the previous 2 tax years)

Also you are presuming that the mother has near perfect finances, who has those? We are all robbing peter to pay paul, even if she did have her tax credits how are they alone enough to support a family she may have debts... council tax, water, phone, gas, electric.. finance company..loan shark.. that's before she's bought food.
It would not be enough to live on.

It irks me that even seeing the film it's presumed the writers have overlooked the bureaucracy involved as well as the complex nature of what it means to be living in poverty in England.