You can't 'reject' a NIP.
A NIP is a warning; it's an advisory and requires no action on the part of the recipient. It requires no response, acceptance or challenge, it is purely, exactly as it name says, a notice.
However normally annexed to / combined with the NIP, is a request for driver details pursuant to s.172 Road Traffic Act 1988.
That is a completely separate form legally wise, and this DOES require action on the part of the recipient; indeed, an offence is committed if the request is ignored, or returned incomplete, unsigned*, without driver nominated, etc.
Moreover and very often missed, is that even if the NIP is defective (through late service, such as past the 14 day limit for instance, or incorrectly filled out etc), then this does not automatically absolve the recipient of complying with a valid s.172 request and providing the details of the driver within 28 days of its receipt, that section of the form Has no LIMITS ON TIME FOR SERVING.
An often made mistake is thinking that the two, an NIP & a Request for driver details, are one in the same, they are not, they are two separate documents.