Ignore her and other posters saying she hasn’t enough to live on, sorry but she has. If she has max loan, your contribution can be zero, you don’t have to give her anything, let alone the £400 she demands.
If halls are 2k/term, she has £900 left from loan, so approx 225 for a 4mth term, plus her earnings, that’s over £600 a month. Plus her bursary. She needs to get real, that is more than most working adults have as spare cash.
Those friends getting more from their parents will also more than likely be getting minimum loan, not maximum like her! I doubt many will have £600+ to live on each month.
DS got min loan, we topped it up for halls fees, then gave him about £250/mth for living costs. So he lived on that £250 and said it was more than enough. In total he got less than the max loan by at least 1k.
In second year his accommodation was less, so our top-up was reduced and we increased his living allowance. He tried to stop us giving him that increase!
He also worked during part of first year, but saved his earnings.
Plenty of parents pay for cars, we did (both v grateful and paid own insurance out of earnings) but generally it’s only parents who easily afford to buy the cars. To have used your redundancy money to buy her a car, MacBook, pay insurance etc, it seems that you feel you have to buy her things and give her money? You don’t!
I too would wonder if there’s drugs or such at play, but only you know her, and know if she is generally entitled. Maybe delay changing the locks at this stage, but send her a text or email pointing out that she has more than enough and that your £100 will stop with immediate effect. Maybe also give her one last chance to apologise and start behaving like an adult, after which you will change the locks. She needs to learn that her actions have consequences.