Personally I actually think it is too late to challenge now as the proposals have been in for a long time now. There will be an awful lot of extra bonus pension sacrifices come March to bring people just below the threshold.
"his really is another disincentive for single parents to get decent jobs"
Not all us single parents find freebies & handouts motivating because they don't represent security. Some of us have enough pride in ourselves to take on the better jobs in spite of it meaning we lose out on various benefits or pay more tax or fork out for childcare and school meals etc. Because what you earn - unless you lose your job - cannot be taken away from you. Personally, I prefer it that way.
I have a very good friend who is a single mum. Back when she was with her husband they had very little money. Every penny was watched. They broke up and now she's significantly better off on her own than she was with him.
They're both still doing the same job. She's working 3 days part time and the rest is made up of tax credits and maintenance. He's not a big earner so it's not like she gets loads of maintenance.
She's even managed to take on the mortgage by finding a company which would take her tax credits into account. Otherwise she could have moved into rented and probably claimed HB.
To me there is something fundamentally wrong with a system where a household with one adult can be left significantly better off on their own working part time hours than two adults, with one full time and one part time.