Leaving aside the obvious legal implications (there's a very good reason why most big NSP's Euro servers are NOT in the UK) I'd imagine there are two main reasons NTL don't do as you suggest:
1. Most of their customers are blissfully unaware of the existence of Newsgroups, and to advertise them would only invite a too close scrutiny of their content.
2. Cost.
It takes a large amount of money to run newsgroups well, and a large dedicated expert team - and this is NTL we are talking about
