Hi Brian
Personally, I wouldn't trust a UK van to hold anything heavier than a bucket of water! I certainly wouldn't think of placing my whole weight (only 8 stone!) on it. Think of all the sealant points that could give way.
Yes, we had a German-built Hobby caravan. Great. No problem standing on the roof of these. Unfortunately, they are so austeer inside and have little work surface so it had to go.
Ann