Claude Opus 4.6 vs GPT-5.3-Codexfor 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.

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.