There are only a few lenders who current have this in the UK. And if used correctly can potentially save a large amount of money. Remember borrowing money should only be done as a necessity because interest is charged and costly. But generally a mortgage is one of the cheapest ways to borrow money and there maybe potentially a lot of equity tied up in the property due to the large rise in UK house prices.
The problem is people take out further loans or borrow extra on the mortgage with out controlling when the money is actually needed. But as soon as the money is raised then the customer is paying interest. A far more efficient system is drawdown. Which means when the extra money is required only then will interest be charged.
For example extra money may have been borrowed on a mortgage to plan for an extension to a property. That extension could take maybe two years to build and costs would be distributed throughout that time, a large completion fee could be due at the end of that time. So to pay interest on money that is not being using until much later is extremely inefficient and costly. But imagine only paying and drawing down amounts from the agreed advance as and when it is required at each stage. Far less interest is paid.
Lenders potentially lose out interest charges but the customer saves money.