Universal Credit reduction when accepting student finance

Hi there, Living in Wales so I understand it may be a little different. F 26 studying with OU. Married, 3 children 5, 4 & 1. My husband and I have a joint claim for UC and my husband also has LCWRA. I suffer severe social anxiety, depression & adhd, I also have a diagnosis of borderline personality disorder and was signed off work a while back but wanted to accomplish something for myself by studyjng with OU. When I first looked into studying I checked with UC as to how our finances would be affected. I hadn't claimed and financial support from OU but was told by UC that whether I made the claim or not our UC payments would be affected but they couldn't tell us how much by. I decided that as they were deducting the money regardless that I would apply for the finances through OU. I was told I would receive a payment every quarter of £1200 aprox. So around £400pm. Our claim with UC was almost £1400 pm including the child element and LCWRA. Since starting the course UC have reduced our monthly payment down to just over £600. We are really struggling since this huge drop with income. It means that UC have deducted money from the child elements and partial from LCWRA. This doesn't seem right to us. Surely it should only be around the same amount deduction that I'm receiving in SF? We have questioned UC but have received nothing more than "this is what it is." We live very rural and need a car to get about. We don't have any luxuries. And with the bills to pay we're literally feeding the children and going without food ourselves. Can anyone please advise as we are really struggling and just can't understand how they can reduce our finances so drastically. Thank you in advance.