UK student loan repayment is income-contingent: you pay a percentage of income above a plan-specific threshold, not a fixed installment, and the threshold differs by plan (1, 2, 4, 5, postgrad).
Because it behaves like a graduate tax that's written off after a set period, overpaying early often doesn't pay off unless you'll clear the balance well before write-off.
Model your plan's threshold and rate against your salary to see your real monthly deduction before deciding whether to overpay.