The amount you can overpay depends on your lender and your mortgage product. Many fixed-rate deals allow overpayments up to a stated annual limit.
If you exceed the permitted amount, early repayment charges can reduce or wipe out the benefit.
Look at your mortgage offer, annual statement or lender online account for the exact rule.
A calculator helps you compare partial overpayments even when charges or limits apply.
Run your own scenario and compare one or two alternatives. That usually reveals the real decision much faster than generic advice.