I also got caught in Dorset, a few years ago, whilst I was still working.
Driving east between Dorchester and Bournemouth, I noticed a patrol car following.
Sticking strictly (or so I thought) to the speed limit, I was surprised when the patrol car overtook and pulled me over.
“Good morning Sir. Do you know why we’ve stopped you?”
No
“What speed were you doing?”
Just below 50.
“That’s why we stopped you. It’s a 40 limit.”

I attended the excellent driver awareness course at Winfrith.
However, I did note that between entering Dorset at Lyme Regis and getting to Winfrith (about 30 miles), the speed limit changed 18 times, with varying signage!!
I, politely, pointed this out on the course and was told that all the signage complied with the legislation in place, at the time of installation!
No excuse, but the signage on the A35 and A31 is a mess and the location of the speed limit areas seem to be regularly altered.
In particular beware the village of Chideock on the A35 westbound.
There is a steep hill leaving the village, which has an uphill second traffic lane.
However, it remains a 30 mph limit all the way up the hill, so don’t be tempted to enthusiastically pop past that lorry.
Bud
4/4 - Stanley