What year did the law change, restricting car licenses to 3.5 tonnes?
It has been in place since as far back as my first license and I got mine in 1996.
How much further back do you need to go? Friends of my mother have a camper and they had to get one less than 3.5 tonnes because of their B license. These people are much older than me and had their licenses from the 1970s.
I have a vague recollection that, when they reviewed the license system (in the late 1980's I think) they removed categories by default but you could retain them if you appealed or furnished a medical certificate or something like that. A lot of people who previously had a heavy van/light truck license (now category C1) just didn't bother as they mainly drove cars.
Even under the new system, some people baulked at having to do a medical. I was in the queue in the old tax office in Chancery Place years ago and a man in front of me was renewing his license. The official said he'd have to present a medical to retain the truck and bus categories but he said he wasn't willing to spend the money so they renewed it with just a category B. Madness on his part.
Just renewed the licence and they took away a load of categories like buses and trucks off the licence. Can't say as I noticed them and didn't reapply for them