This is starting to get needlessly bogged down and complicated again...
As Smiffy says above, this is going to have next to no effect on the general public as far as towing things, even 4x4's on a trailer behind you. It's for those who tow trailers in the course of work or commercial activities using vehicles classed as commercials, not the PLG type that a Fronty falls under.
Baz: Your funeral director that's mentioned, quite correct that he needs an O-license, as he's driving a commercial vehicle and he's working for reward - namely whatever fee he has agreed for providing his services as a funeral director (I'll turn up and provide this service, in exchange for a price of xxx. It's a legal contract).
Dav: The chappie pulling his 4x4 special for competition use doesn't need an O-license because, prize money or no, he's not doing it for hire or reward but for personal or pleasure use. If however he entered into lots of events with the intention of winning any prize money as his primary income then that would likely be a different matter.
For the rest of us, towing another 4x4 behind you is exactly the same as towing your caravan on holidays, or a trailer full of rubbish to the skip, or hiring a 7.5t truck to move house with.
As long as you stay within the legal weight limits for the vehicle / trailer combination then you're fine; your mate saying "can you do such and such for me with your trailer, and I'll pay for the fuel / buy your lunch" won't make any difference.
Even Woodwizzard is unlikely to have a problem with log deliveries because a Trooper is a PLG, so as long as he's honest with HMRC, no problem.