The long and the short of this is that if you decline the course which will almost certainly be offered, you will be prosecuted in court.
There the police will produce evidence to show that your vehicle's speed was detected using an approved device. They will also be able to show what the prevailing limit was at the time. Your task will be to cast doubt on that evidence and from what you say, all you have to help you is "...but I am sure that I wasn't speeding and was sticking to the displayed speed limits".
It's not too hard to imagine which way the court's decision will go.
In the event you are convicted you will pay a fine of half a week's net income, a surcharge of 40% of that fine and prosecution costs which will be in the region of £650. So no change from £1k for most people.
You could contact National Highways to check the prevailing limit. They should have a log of variable speed limit impositions.
I take it you are the Registered Keeper and that your NIP arrived within 14 days?
