Quote from their website:
"Vans carrying COMMERCIAL GOODS FOR RESALE must be booked through our freight service. Vans carrying non-commercial goods exceeding 3 cubic metres in volume must also be booked through our freight service. "
I mostly carry camera and broadcast rentals, temporarily shifted from one company to another for productions: football, concerts, TVshows, golf,... And NEVER had any questions asked so for (I've been doing this for over ten years now, twice a month on average).
Their "commercial goods" regulation is a company rule, in my opinion simply meant to get as many vehicles as possible to pay as much as possible.
To avoid confrontation, I ALWAYS pre-book online and check-in at an unmanned, automated booth. No questions asked.
Customs nor police ever seem to care what I am carrying, as long as it's nothing illegal. Yet they regularly check or at least ask about my load.
Every now and then I get to take new camera gear or other audiovisuals from UK to EU. Even in those cases I never ever had any problems with customs or police.
To me it's crystal clear that this is a commercial rule aimed at making more profit.