Self -cert mortgages are for people who for one reason or another can not prove their income. This is often the case with some self-employed people, people with multiple jobs, relying on commission or with income sources not recognized by the banks, like tips or cash-in-hand. Self-cert mortgages were also popular when speed of offer was important as they were often considerably quicker. They have always been a bit more expensive and generally required larger deposits.
Due to the current crisis self-certs are quickly going out of fashion. Lenders are now charging much higher rates and the deposit required is 25% on average. As a result less and less customers are relying on self-certs.
It is not advisable to lie about your income to get a mortgage as there is a risk you could end up with mortgage you can not afford.