100% buy to let mortgages are slowly fading away in the liens world. Once upon a time, they were very good options for investors looking for a property to let. However, as time went by and interest rates rose, investors realised that 100% buy to let mortgages were not what they seemed to be. The main disadvantage of this liens is that, should there be a fall in the market value of the property, you'll end up paying a lot more than you should. This is called "negative equity".
Borrowers of 100% buy to let mortgages may be asked to pay a mortgage insurance in case the property value decreases. Some companies do not ask for insurance in that situation but they do increase the monthly payments of 100% buy to let mortgages.