Apart from the link John posted, any general Internet search on "Internet Usage Guide" or similar, brings up any number of results that should help any prospective customer decide on a limited or unlimited package regardless with which provider.
You are assuming that they are not Internet savy.
For novices signing up for the first time, they would be unable to give information to BT about what their Internet usage normally is or what they would be using the Internet for so it would be difficult to give them accurate/relevant information.
If you go onto the BT broadband package page select "I'm new to BT" and follow the links "Help me decide" it gives a guide to what broadband package might suit the user.
Just a thought, if they are ordering BT Broadband online it might be fair to assume that they have some knowledge about using the Internet or at least have access to the Internet through somebody who has some knowledge and would be able to assist them with a simple search.