If you don't reply to the 'NIP' (well the S172 request on the same piece of paper) then you commit a more serious criminal offence than the speeding (6 points and typically no change from £700).
Also as you won't have confirmed who was driving then no offer of course or fixed penalty could be made.
So yes, I would reply.
You have no right to see any evidence at this stage, you could ask for a 'copy of any photos to help confirm the drivers ID' BEFORE naming the driver, but be aware that it doesn't give you any extra time in which to reply.