As I understand it an overpayment goes directly against capital, the expected payment is a mix of capital and interest, so overpaying is more efficient at paying down the capital balance than paying a higher expected repayment on a shorter term. Plus you can vary if you need to.
Cash is fungible, right? Any cash in excess of the monthly interest charge is going to reduce the outstanding principal balance £-for-£. It doesn't really matter whether you call it a scheduled payment or an overpayment.
How does that work? If you extend the term but overpay isn’t it just the same as having a shorter term in the first place?