GPT-5.6 Luna vs GPT-5.6 Sol for everyday maths

Mortgage interest is charged on the amount you still owe. An extra £100 paid early reduces your balance sooner, so you avoid interest on that £100 for many more years. Those savings can also reduce future interest, because each later interest charge is based on a slightly smaller balance. In the final five years, the balance is already much lower and there is less time left for the overpayment to prevent interest, so the same £100 saves far less overall.

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.