The final recourse for a mortgage lender is to repossess and sell your house to recoup their money. It’s not something they like to do, but it represents the bottom line for them if you were to fail to repay their money.
Selling your house is unlikely to net them the full market value. They are not going to put in the same level of effort that you might when selling it (no smell of freshly baked bread and perfectly arranged flowers for visitors) and are looking for a quick sale. On a £280,000 home, for example, the mortgage company might only get £250,000, and once their administration costs and other expenses are accounted for, that figure drops further.
If the mortgage represented 100% of the house (i.e. you had no deposit and they had leant you £280,000) and you defaulted in the first few months after paying £1000 per month, they will have made a loss with their investment. This would be the case if the loan to value ratio (LTV) was 100%.
With a 10% deposit (£28,000), the LTV would be 90% and they would scrape by with perhaps a minor loss depending on the cost of their administration.
A 20% deposit (LTV 80%), would mean that on repossession and sale, the lender still makes some money. In this case, they’d have provided £224,000 and would claim back £250,000.
The higher the LTV, the greater the risk for the mortgage lender.