Outstanding mortgage is say £200,000
Bank repossesses property and puts it up for sale
I make the highest offer (£120,000) and get a cheap house
It costs Bank £10,000 to repossess, market and sell
Outstanding debt (which the Bank will sell on to Company X) = 200,000-120,000+10,000 = £90,000
Company X will pursue the outstanding debt ... with further charges
I and the Bank are the only happy bunnies