Perhaps a little explanation will help you understand this:
At present, unless you plead guilty, you cannot be convicted of speeding as the police have no evidence that you were driving.
They will, however, be fairly confident of seeing you convicted for "Fail to Provide Driver's Details" (FtP). But unless you plead guilty, this will involve a trial and, like you, they have better things to do.
However, if you simply plead Not Guilty to both charges, they will proceed with the one where they are most confident of success (FtP). You don't really want that (because it is a far more damaging conviction) and they don't really want that either because it involves the bother of a trial.
So, you enter NG pleas to both but make it known that you will change your plea and admit speeding if, and only if, they agree to drop the FtP charge.
So the end result should be they are happy because they haven't had to bother with a trial but have still secured a conviction. You are happy because you are only convicted of the offence you actually committed rather than one which was possibly caused by items lost in the post.
