Getting a UK postal address

What is the bare minimum information needed to determine a user's full UK postal address?

From:

• Address line 1
• Postcode

there is generally enough information to determine the user's town, city and other necessary information required to send an item by post.

My question is; are there any exceptions to this rule? I building a free fabric sample request form for a website and simply asking for the first line of the address and postcode would be great.