Once the bailiff is inside your house after entering peaceably, they will firstly search your home. They will not take items on their first visit, but will make a list of what they intend to take and sell (this is called seizing).
Once in your house they have the right of access to all rooms, and can use force to gain access into other parts of the property.
They can call again at a later date and enter your house without your permission, forcefully if necessary, to remove and impound your goods.
However, they can only seize and impound goods to cover the debt and their fees, nothing more. When seizing goods they must leave the premises safe.