GPT-5.6 Sol vs GPT-5.6 Terrafor everyday maths

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.

Early overpayments cut the mortgage balance when it is highest, so every future monthly interest charge is calculated on a smaller amount. That saving then compounds over many years: less interest builds up, and more of each normal payment goes toward the loan itself. In the final five years, there is less balance left and far less time for reduced interest to accumulate, so the same £100 overpayment has a smaller overall effect.