GPT-5.3-Codex vs GPT-5.6 Solfor everyday maths

Because early overpayments cut the loan balance sooner, and interest is charged on whatever balance remains each month. So that £100 doesn’t just reduce debt once — it keeps reducing future interest year after year. In the final 5 years, there’s much less time left for that “interest on a smaller balance” effect to build. Same £100 paid late still helps, but it has far fewer months to create extra savings.

Mortgage interest is charged on the amount you still owe. Overpaying early reduces that balance for many years, so you avoid interest on the overpaid amount throughout most of the mortgage. It also means more of each later payment goes toward clearing the remaining debt rather than interest. In the final five years, the balance is already much lower and there is little time left for interest to build up, so identical overpayments save far less.