I'm a freelancer doing work not totally dissimilar to Jack Monroe (but not food-related). I've never worked on a project that didn't have a deadline, agreed fee and clarity on when you'd get paid (i.e. Terms and Conditions).
Do I sometimes have to chase payments? Yes, absolutely. Are projects sometimes delayed? Sure, that can happen and it's annoying. But you also get paid £5,000 or £10,000 in one go (I suspect with her brand collabs, she gets the odd one-off payment that's higher). Most successful freelancers appreciate that that's not their monthly salary, but this money has to be spread out over the next X number of months, before the next invoice is due and a percentage has to be set aside for tax. That's why starting out can be really hard, but once you are set up, with a large public platform and a relatively high profile, lots of contacts and brand collaborations, life should be easily manageable. If it's not, there's a problem and you either can't make it work and should look for employment or, perhaps, you aren't the one to give budgeting advice to others, ya prune!