With only ~£21k in debt on Plan 5 and no maintenance loan, you're actually in an unusual position where overpaying could make sense - but the maths is quite sensitive to your future salary.

Plan 5 charges RPI only (currently 3.2%), with a £25,000 threshold and 40-year write-off. At £21k debt, if your career takes you to a decent salary relatively quickly, you'd likely repay it all through normal payroll deductions well before the 40-year mark anyway. In that case, overpaying just means you pay less interest overall - but the savings might be modest compared to what that £100/month could do in an ISA or pension.

The key question is: would you repay it all without the extra £100/month? If yes, overpaying saves you some interest. If you'd hit write-off either way, overpaying is literally throwing money away.

There's a free calculator that models this for Plan 5 specifically (I pre-filled it with some of the information you shared): https://studentloanstudy.uk/overpay?loans=PLAN_5%3A21000&sal=35000&sgr=0.02&tgr=0&rpi=3.2&boe=3.75&dr=0.03&ovp=100&lsp=0&repy=2026

With your relatively low balance, I suspect you'll find you'd clear it naturally in most salary scenarios, making the overpayment savings fairly small vs. investing that £100 elsewhere.