The police officer has loads of evidence. They will have a speed reading and a statement detailing their observations. You would need to have a defence that you weren't there and explain where you were and introduce enough doubt that it wasn't you (which isn't easy). Simply saying the officer might have made a mistake isn't enough.
Im not saying you should go to court and just say "you're wrong", im saying that a statement based off a basic speed measuring device on a busy road is pretty flimsy by modern standards and the public has every right to be able to defend themself about wrongful accusation.
Lets say, for example, you get a NIP through that says your can was going 42 in a 30 zone, but you cant ever recall speeding in the area stated in the NIP. Should you just roll over and take it despite the fact that there is a realistic chance that there has been some kind of error?
No bloody chance, request further information
to help identify the driver. Not evidence, just assistance, as step one, whether you're contesting the ticket or not, is for you to actually identify the driver. They don't have to provide it, but there is little reason for them not to do so as they actually want you to identify the driver... so they generally do... if they have the photo.
If there is nothing they can provide then I think you have grounds to appeal either informally or formally, and if that falls on deaf ears, consider taking the matter to the courts, because it's quite likely they have zilcho and it is up to them to prove that YOU were driving AND you were speeding.
That's exactly why you would generally get pulled if there is an officer monitoring with a basic speedgun, and why most of the modern system take a picture or video with the driver in shot. They want that NIP to be assigned to a real person.
If they are regularly calibrated - which they should be and there should be a log - then you've got a mountain to climb. The devices need to be Home Office approved and so this will be a non-starter in all but the most exceptional of circumstances.
They should be yes, but the Police are just as open to mistakes as you are and if you do take it to court, you will be presented with all the evidence they have, including logs, software version and calibration records. If an officer is sat with a speed gun writing down number plates and sending off NIPs, how detailed do you think his statement is going to be?
Anyway... this is a pointless argument as the overwhelming majority of speeding offenses are with cameras, it's far more efficient/profitable for them. If there is a dude with a speed detector and it doesn't have a camera, or a follow up car/bike... my guess is that it's more of a deterent.