I think ntl are right to an extent
I think you should have to prove who you are to be able to add services to what you already have (Data Protection etc) as you would soon be pee'd off if you found you had a range of services that you hadn't asked for (if 'someone else' had rung up & added them to your account for no good reason)
But (as usual) they have communicated this to you in a very poor way, & yet again treated a customer who is trying to put money in their pockets very badly.
This is not the way to do business ntl.
I would do the same as you-give my money to an alternative provider if the one I was with treated me like that (& that doesn't just apply to ntl as I don't have their services-if Pipex/BT treated me like that, I would take my custom elsewhere)