The current edition of Yellow Pages mentions – for the first time ever – that it's possible to cancel the delivery of the book. BT has now followed Yell's lead. I've just received a copy of the new phone book, and under 'Phone Book Customer Services' it includes this message:

Opting out of receiving The Phone Book

To opt out of receiving The Phone Book
Freephone 0800 833 400
Mon–Fri, 8.30am - 5pm

Another small but significant step. In 2008, BT told 192.com's Say no to Phone Books campaign that the company would ignore any opt-out requests, apparently claiming it had a legal obligation to deliver the book to every address in the UK. Let's hope the company will soon start making it easier to cancel the book (it's not yet possible to do this via the BT website) and that they'll stop that nonsense about demanding the phone number of people who want to cancel the delivery of the book.