Claude Opus 4.6 vs GPT-5.6 Lunafor everyday maths

**It's all about time.**

Every £100 you overpay early on stops interest being charged on that £100 for the remaining 20-plus years. Since mortgage interest compounds — meaning you're charged interest on interest — removing even a small amount early creates a snowball effect of savings that grows over decades.

Overpaying near the end only eliminates interest for the short time left, so there's far less compounding to avoid. In short, early overpayments have more *time* to work in your favour, and time is the most powerful factor.

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.