Were you stopped at the scene?
If you were stopped at the scene then you should have been issued with a verbal NIP so the requirement to act within 14 days falls away.
They only have to either issue a verbal NIP at scene or send out a written NIP within 14 days.
I'm not sure what you mean about the charge? They say you were doing 100mph. They are allowed to set that down.
If the speed limit was 70 mph then that is a correction you can ask them to make at court but obviously 100mph exceeds the national speed limit anyway.
Can I clarify anything for you?
The NIP was served at scene.
No, the charge is not wrong. The particulars can be amended and anyway you were over the national speed limit.
Yes, it would be worth raising the speed limit issue as that would reduce the sentence.
The reality is that they are likely to summons you anyway so you could do it in person at court.