If you read the NIP carefully, you'll see that it is actually a Magistrates Courts Act Section 9 statement. So by filling it in with your details and then signing it, you are sending them a signed statement admitting you committed the offence. However, as pointed out in the article, there is nothing in law to force you to sign it, only to give the details of the driver/rider (
http://www.legislation.gov.uk/ukpga/1988/52/section/172). Other people have tried sending the requested details in on a letter rather than using the form. You have complied with the law by giving the details they have requested but not in a form that can be used in court.
However, it can backfire as I found out a few years ago. I sent my details in as a letter and still got a summons. In fact I got two, one for failing to provide the information and another for speeding even though they had no evidence of who was riding at the time. I was found not guilty of failing to provide details as it was obvious that I had (prosecution produced my letter as evidence that I hadn't supplied the information and I produced the same letter as evidence that I had, magistrate took one look at it and dropped the charge) but that was then used as evidence for the speeding charge. Got 3 points and a £120 quid fine instead of £60 if I had simply rolled over and taken it. Entertaining way of spending an afternoon though and I was subsequently told by a solicitor that I should have asked for the two charges to be heard separately with the speeding charge (where they had no evidence of who was driving) heard first......
One other point to remember. Information must be laid before the court within 6 months of the offence (which will result in a summons being sent out within a couple of days). So if, by slowing things down by not signing, not fully completing the NIP or writing asking for copies of the tape or anything else you can drag it out for over 6 months, then a court case won't happen. If you do nothing, as happened with Nick, then it will go to court (within the 6 months), you will be found guilty and when you don't pay the fine because you don't know anything about it, a warrant will be issued. They give you 28 days to send the NIP back in so do it, or any reply to it, on the 27th day.