There are some technical bits - like they do not have a break-away cable. Easily retro- fitted though to keep Plod happy. They do not have an "auto-reverse" brake, on the modern ones you have to get out and operate a locking push-button to stop the brakes coming on if you reverse any thing other than gently. On the oldder ones the same effect is achieved by dropping a metal u shaped sleeve onto the brake piston. These issues mean that they would not pass the Regs if made now. The later construction requirements are not retrospective so they are OK to use, just like an older Landrover would not meet modern construction regs - airbags etc. I read some nonsense on one forum about how they were illegal because they were all 24v. Utter rubbish. If you find one marked up as 24v ( as towed behind an FFR radio vehicle) all you need to do is change the bulbs to 12v, hardly rocket-science.
Note there are two main types, the older "narrow track" version with four solid walls and the later "wide-track" version which has a drop-down tailgate and is bigger/heavier - but more plentiful as MOD only finished selling off its stock last year. A "narrow track" will look good behind an ex MOD Series vehicle.
The License thing may be the stumbling block for the younger generation. My license means I can tow a loaded Sankey - others may find they are limited to 750kg Gross weight trailers, which with a wide-track Sankey would only allow you load of just under 250Kg.