Those in negative equity may need a mortgage with an LTV over 100%. Negative equity is when your house value drops below the mortgage amount.
For example: your mortgage is £200,000 but your home has decreased in value to £180,000. You would therefore need a mortgage with a 111% LTV.
To get out of negative equity, you could use savings to bring the LTV down to 100%, or lower. There are very few mortgage products available for LTVs over 100%, and the interest rates tend to be very high.