The debate here is not about whether it pays to go back to work when childcare costs are high. OP although not said directly, has made it clear that ultimately, she much rather be at home with her baby whilst being able to study than working. I very much doubt that even if she was £100 better off a month, she would have gone back to her job.
If 12 months of tax payers money keeps a roof over my head and food in my family's belly during that time....I can only thank you all from the bottom of my heart
And if indeed, it turns out that you only claim for 12 months, because you'll be in a much better job after having trained in a year's time, I would totally agree with you. The problem is that you are probably a bit naive in thinking that this will happen. It might, but the more common scenario goes like this.
You start your course but realise that it's actually quite hard to study whilst looking after a baby ft, so in the end, it doesn't take 12 months but 24 months. During that time, your child has started some hours of childcare and after 2 years of them being stuck to you, having some hours for yourself is a real treat. You apply for jobs in your new field but are not getting the response you expect because they wanted trained people who also have experience, so even when you get interviews, you don't get the job. That and the fact that you are so attach to your child, you don't want them to go to after school and breakfast clubs, and you don't want to commute too far, and you want the holidays, so you are restricting the jobs you can apply for. Then there is the fact that you've been out of work for 3 years and it has become unfamiliar to you and quite scary. You are very happy as a sahm, spending quality time with your child, having a few hours for yourself, and you've adjusted to living on a reduced budget with tax credits. You love being a mum and wouldn't mind having another one, also a sibling for your child, and of course, that would mean more UC.
That's why people react the way they do, but it is rarely just a case of claiming for 12 months but in a very large majority of the case, it becomes claiming for many years as it becomes the norm.
If you do go on and act on your plans, then good on you and indeed, make the best of the 12 months.