This is a mortgage where your monthly repayments only cover the interest on the loan and you are not paying back any of the capital borrowed unless you have elected to make overpayments.
It is therefore very important that you have a repayment plan in place to repay the capital at the maturity of the loan. This could be an investments and savings plan, overpayments, personal savings or other liquid assets.
If at maturity, there is a shortfall in your investments and savings plan, you will still be liable to repay any amount of capital outstanding.