is that all broadband suppliers - no matter who they are, or what they charge - are buying their broadband services from BT wholesale Ltd.
This means that they are all going to be paying much the same, and anyone who resells to you at monthly charge that seems unrealistically low is likely to be on a margin that's so narrow there isn't room for any customer service whatsoever.
The bigger service suppliers like BT OpenWorld, Pipex, Demon etc., are all charging a figure that's a lot higher than £18, so you should see a patterm emerging.
My advice is to stick to someone who is a well-known player in this field. My own home connection is with BT OpenWorld at £29.99 a month, and I wouldn't dream of changing.