I had the mis-fortune of having to do a Driver/Speed Awareness Course a few months ago.
As fascinating & interesting (NOT) as the course was, some of the information that the guy running the course told us all during lunch break, was enlightening.
As Phil.P and Roger S said, some of us have what the course guy told us is a "Grandad Licence". This entitles us to drive up a 7.5 ton lorry even though we have never taken a test to drive one. Apparently, if you passed your test after a certain date (Cant remember what the date is), then you would have to sit a test.
One thing that did come as a shock to me was what he told me about pulling a Trailer. I own and regularly drive 2 Vans (Renault Trafic & VW Transporter) and I also had a small 4 wheel enclosed Box trailer, about the size of a small horse box. He stated that when loaded and being pulled by either van, the GVW was such that either of the 2 vans need to be fitted with a Tachograph........!!!!!!?????? and should I get pulled up by Mr Plod, then I would get nicked if no Tachograph was fitted to the vans.
The next day, I looked into it and, obviously, he was correct.
The cost of having a Tachograph fitted to either or both vans was astronomical, so I had to sell the trailer and make do without it.