With a repayment mortgage your monthly repayments cover both capital and interest on the loan.
As the term continues, the amount outstanding on the loan reduces so the full amount of the loan will have been repaid at the end of the term as long as you have maintained payments.
No other repayment vehicle is needed and it avoids the risk of investing (e.g. in the stock market).
If you remortgage, you may be tempted to extend the end repayment date in order to lower your monthly payments. However this means that the amount you repay overall increases over time.