I think Derb is exactly right. I have also come across quite a few with issues, and the difficulty for any purchaser is knowing how to avoid the problem vans if you come across one. This applies to any van - older NZ ones that have remained as touring vans and are regularly used and well maintained would be my preference, but again you have to be careful. Australian vans are hit and miss, there are many variations in construction quality.
Many of the older UK caravans (if not all) have poor insulation. This was very apparent when Barrons first set up in Taupo about 14 years ago or so, and for a while sold UK vans alongside NZ vans (Oxford). When you entered a closed up British van in Summer, it was invariably hot and stuffy. When you entered an Oxford, it was quite liveable. The difference was remarkable. The attached photos of a 2005 UK van that literally fell apart in the container during transit here shows just how flimsy the construction can be, and the very limited insulation, if any in places. You can also see where it has been leaking - this is not isolated to UK vans and a thorough moisture test would be advisable.
UK vans with ultra light aluminium sheet cladding dent very easily and can be difficult to repair properly - the original profile can be difficult if not impossible to get. Many also come in with existing dents, so it becomes a matter of how tolerant you are to that cosmetic feature.
A friend of ours has just purchased a small motorhome as he got sick of maintenance issues with his early 2000's English van - both sidewalls were starting to droop - I'm not sure if he was worried he was about to lose headroom in the bathroom
Claims that UK vans have all the bells and whistles that NZ and Aussie vans don't have make me wonder exactly what those are.
In the last 4-5 years, there has been a change of construction and modernisation of UK vans, hopefully for the better. So this is an era thing. Personally, I would be very careful with any second hand van to get it checked, but especially with an older UK one.