If you feel the need to contend the matter, you need to act quickly.
NIP must be served within 14 days of the offence. Check the postmark of the envelope against date of alleged offence. If its more than 14 days respond with a letter advising the camera partnership of their error and await a reply.
If its within 14 days (and thats postmark remember, not the day it dropped through your letterbox) proceedings are deemed commenced.
As to the speed regulatory signs being missing or covered up by vegetation? Worth a shot but, in all honesty, doubt it may proceed. If its a one horse village with one road in/out then might just work. If a larger town with several routes in/out? Doubt it.
Faulty camera? Pub talk, or based on fact? If it was known to be faulty by the Camera Partnership, why send out the NIP's? Again, worth a try but dont hold your breath.
Overall, I think the last line of your initial post kinda sums it up.
If the signage technicallity is legitimate, then am I leaving myself open to being shafted for all else?
If you are contesting the matter, would suggest not returning the NIP - yet - simply aknowledge its receipt and enclose your reply on whichever point floats your boat.
Take care to ensure that you are in a position to return the completed NIP within required timescales (normally outlined upon it) otherwise the matter may well be escalated.
Ultimately, irrespective of the reasons you perceive as mitigating circumstances, it appears that you overtook on solid whites, at excess of the speed limit, because you were unable to stop in time to prevent yourself going up the harris of the car in front - which indicates you and he were perhaps playing tig and you were too close to react to their brake testing you.....