I'm not sure about the Isuzu pick-up but the old Diahatsu Fourtak short wheel base can tow 3.5 tons, I'm sure there will be others.
You'll be aware that HGV's are subject to Operator Licensing requirements, ie they are subject to tachograph rules, they must have an operator's license, and whoever is "responsible" for them must hold a professional CPC. This is to make sure the vehicle is maintained properly, and that the tacho rules and driving/working hours are complied with.
However these rules aren't only for big HGV's, they actually kick in at 3.5 tons gross train weight (which is now called MAM Maximum Allowable Weight or something). That's why you see lots of single wheel base 3.5 ton gross Sprinters around, not the double wheeled 4.5 tonner ones.
So, in theory, if you're towing a heavy trailer with a Sprinter you'll be over 3.5 tons so you need an Operators License. But, every man and his dog seems to tow a mini excavator on the back of a Transit, so it looks like the rules aren't enforced much. I think there's different rules depending on whether it's a goods trailer or not tho, and I also think you can use log books instead of tacho's, depending on what you're towing, how far, and how often.
MAM also becomes important. Say for example a 3.5 ton gross Sprinter can tow 3 tons (I can't remember exactly). You'd think the MAM would be 3.5 tons + 3 tons = 6.5 tons. But this isn't necessarily the case as the MAM for the van might be just 5 tons. So your van would need to be empty (at 2 tons unladen) so that your combined weight of van + trailer is 2 tons van + 3 tons trailer = 5 tons MAM.
There's more to it all, and my memory is rusty on some of it, but check it out properly if you don't want Plod on your back.
