The best way to explain a Let to Buy mortgage is using an example case.

Mr & Mrs Smith are living in a property that they currently own with or without a mortgage.
They would ideally like to buy a new property to call their home and rent out their current property to tenants.
The mortgage required to re-mortgage their existing property as a rental property is called a Let to Buy mortgage.

This funds from this re-mortgage could then be used as a deposit to purchase Mr & Mrs Smith’s new home and a new residential mortgage could be taken out secure on their new home.
The main difference between a Let to Buy and a Buy to Let Re-mortgage is that with the former the property that the mortgage is secured on is/was the client’s residential home that they lived in. A Buy to Let re-mortgage is when a client wishes to re-finance an existing rental property.