Even if you have a defence your reply to the NIP is not the place to say you want to defend it. The NIP asks you only to identify who was driving. Nothing else.
Failing to return the NIP and provide the information within the deadline is a serious offence - more serious than the speeding charge. As it says on the NIP, not identifying the driver can result in £1,000 fine + 6 points on your licence. With luck 38mph in a 30 mph limit should get you an offer of doing a Speed Awareness Course, so no conviction, no fine and no points. As long as you haven't already done the course within the last 3 years.