The same law applies for vehicles in uk. If it is taxed as a commercial vehicle, whether it's used as such or not, then the speed limits are lower.
I found this out one week after passing my driving test many years ago as I got stopped for exceeding the speed limit on a D category road in a little ford Anglia van. I was doing 55 in a 60 but due to the tax disc I should have only be doing 40.
I asked the magistrate who was 'old' if he knew of this law. He told me that it was nothing to do with me what he knew or what he didn't know. £40 fine and an endorsement
£40 was 4 weeks wages then >:(
Ignorance is no defence anywhere ::)