Claude Opus 4.6 vs GPT-5.6 Terrafor 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.

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.