The quick answer
This is used in a pre-pack administration to prove that the person buying the assets and business are viable themselves and will not fall quickly into insolvency.
In more detail
A viability statement is a requirement of Statement of Insolvency Practice 16 which covers all the extra reporting rules and requirements to satisfy a pre-pack administration has been properly used.
There is no exact form of a viability statement but usually a cash flow projection showing that funds are available and can support the working capital needed for the new business is sufficient.