Best thing is to go back there and take actual measurements if you can, but it can be an expensive day in court if you challenge and lose. You may be lucky and get the standard FPN £100 and 3 points.
Car insurances don't seem to worry too much about speeding points these days.
The Driver CPC courses vary in content, but those I've been on have had a session regrding speed limits. One notable speed awareness course I went on the chap lecturing shed a picture of a national speed limit sign on a 2 lane road and asked "what's the speed limit here?" He wasn't happy with the reply "what vehicle am I driving?"
Court isn't somewhere I want to go for the reason you said, very expensive. Still waiting to find out my punishment.
I've been driving LGV since 2018 so have only done one 35 hour CPC refresher (I did it over 5 days, never again) I don't remember covering road speed limits but obviously that's not a defence for my actions.
You are right about "what vehicle am I driving" but also I've spoken to car drivers that think the NSL sign (white with black line through) that think it means the limit is 70 on a single carriageway
