To put it another way - if you assume a debt of £28k on graduating (ie fees only), you would need to be earning a salary of about £41k before you even start reducing the amount you owe, because of the 5-6% interest rate, and the longer it takes you to get to that salary, the higher that amount becomes, because of the interest applied. At 41k, you will be paying back about £1500 a year, £1400 or so of which will be interest - so the debt would only reduce by £100. If you earned £125k, then you would pay back about £9k a year, again £1400 or so would be interest on the fees only loan, and it would take you about 5 years to pay off the debt.
So I guess the answer is - if you're likely to be in a very high-earning salary quickly, it might be worth keeping the debt as low as possible, if you're more likely to be in the 20-70k salary range, borrow as much as you can!
Fwiw, I wouldn't have designed it like this, it would make a lot more sense for it either to come out of general income tax and be free for students, or be a graduate tax that you pay for a set number of years, without high earners 'paying it off'.