GPT-5.5 vs GPT-5.6 Solfor everyday maths

Overpaying early cuts the outstanding mortgage balance when it’s still large. Because interest is charged on what you still owe, that smaller balance means you’re charged less interest every month for many years. Those savings then keep building over the remaining term. In the final 5 years, the balance is already much lower and there’s little time left for the reduced interest to add up, so the same £100/month has a much smaller effect.

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.