It looks like they're not really made to Uk law spec. Max GVW of the 300cdi van version is 3560kg, and payload is 1050kg. I suppose this could be plated differently for the UK market? Otherwise it's a goods vehicle license for the sake of 60kg!
I'd imagine Merc would have that sorted. But then the pickup wasn't aimed at the UK market, so who knows!
Edit... And not really a derail. It's all info that interests me. Your question prompted me to look at the weights.