I suspect there are two slightly different issues in this thread.
The first post is about the inability at the moment for many customers on the Flex package to add or remove channels. Trying to add gives the error code the original poster referred to. On the other hand others have had cancelled channel packages removed from billing but not from their box. Whilst this has happened intermittently over the past two years, it does seem more common at the moment, but customer services seem unable to deal with it.
The second issue is around special offers. Certainly in the past they have been package related, not type of box related. If one is on the Stream box with a Flex package they have not been available, if on one of the standard TV packages then the deal is available. I've experienced that in the past with a TNT Sport offer that wasn't applicable to the Flex package I had, which took weeks to sort out.
If Virgin Media are emailing promotional offers that do not make such restrictions clear, then in addition to the OFCOM regulated complaints procedure that ultimately goes to the Communications & Internet Services Adjudication Scheme, they are also potentially breaching ASA rules.