Commercial vehicles with a gross plated weight of more than 3.5 tonnes (or, if there's no gross plated weight, an unladen weight of more than 1.525 tonnes) require an operator's licence from VOSA. That licence will specify the 'operating centre' for the vehicle. When not in use, vehicles should be parked at that operating centre where it's practical to do so.
Lighter commercial vehicles can always park anywhere that cars can (unless, of course, there are specific restrictions in force on the relevant road).
Chris