The Post Office is responsible for your parcel from the moment it is picked up. If home delivery is not deemed possible by the delivery person or if the address is not accessible, the delivery person will have the option of leaving the parcel at a collection point (often the nearest post office) instead of the home address.
A second delivery will be possible* to your address if you are unable to collect the order left at the post office (checking beforehand that your address is accessible to the delivery person).
*This will not apply to Christmas offers. Find all the information here.