The actual maths is not something I am prepared to spend a morning doing because it would imvolve deep research into exactly what interest rates have been applied over the last 3 years. However, I can do ballpark figures to show you roughly what they will d. If interest rates were just at 5% throughout her debt would already be just under £38k and if it was 2% throughout it would be around £36k so it will be somewhere between these two.
If you pay her year3 fees & maintenance without increasing the loan, it will go up by another 500-600 ish. If she gets a further year of fees and maintenance loan it will end up between £51k-£54k.
If she gets a job in autumn 2026 paying around £30k her loan repayments will be around £450 per year, no matter how much she borrowed
If she didn't get the final year loans, interest will be acruing at some amount between £700 and £1700 per year so lets take an average and call it £1200 in annual interest. So her loan debt will be going up not down. If she has a 'normal' rather than highly-paid career she will just experience the repayments as a tax. She would need her graduate first job to be paying at least £38,500pa just for payments to equal interest, and would need to rapidly rise to earning at least £50-£70k to have a hope of ever having paid it off, but that might be feasible.
If she takes the additional year of fees, gets a £30k initial job and has a perfectly respectable but never particularly high-paying career, she will be no worse off than if she didn't take the additional loan. Her repayments will be the same. She will be no richer, but you will be poorer. She would in this scenario need her first job after graduation to be paying at least £46,000pa for her repayments to exceed the interest being accrued.
In summary - if you are really confident that she will step straight into a high paying job after graduation and will quickly rise to higher-rate-tax-band levels of salary within 5 years and will never take a lengthy maternity career break or go part-time for childcare reasons, then yes it might save a small amount of money for you to take the hit. If you can't be certain of that then it's not worth it, you will probably not save her anything.
So the question becomes - why does your (theoretical) extremely wealthy and successful highly paid offspring with the meteoric career trajectory need your help? Given that you can't guarantee that she won't end up on one of the other less-well-paid life paths, maybe you should hang on to your money to help those potential-future-versions of your DD rather than spending it now on the potential-version who doesn't really need your help?