When I signed up for NTL Broadband back in 2001, I was told by the install engineer, to call CS and give my MAC, which I did, and I was online. I was NEVER told I needed to register, so I just used my old ntlworld 1-1-1 dial-up mail account.
A year or so later when I wanted to change my speed, I tried the ntlworld speed upgrade; but that failed, because, apparently the modem was "provisioned" but I was not registered for BB.

At the time there was a way of registering a provisioned modem, using a special serial number, so I did that (as advised by tech support) and all was fine

HOWEVER; You can no longer do that type of registration, so call tech support, hopefully you'll get a "good un" who'll create you a PIN and get your MAC nuked (only tech support can nuke) which will allow you to register @
https://autoreg.autoregister.net/cgi/cablereg with serial ntl ct8 cj486
If Tech support say they cannot create a PIN... call sales, I think the number is 0800 083 1234 and ask them, you'll find many of the other CS depts will be unable to do this (lack of training) but Sales do it all day, so should be a breeze for them.
Hope that helps.

What the hell am I doing up this time of night? Bedtime methinx!