This really depends on where in the world you're going and what type of cover is available in that country. If you're going to a European Economic Area (EEA) nation, then your UK insurance is valid for twelve months after moving. This means it's possible to drive across Europe without taking out additional cover, but if you do need any repairs or medical treatment whilst away, this may have to be paid for upfront and claimed back from the insurer upon return home. Other locations such as North America are almost impossible to get insured no matter what type of vehicle is being driven, so this would definitely require separate overseas cover before travelling!
Hopefully, this has answered any questions you may have about van insurance, but please feel free to get in touch if you still have any queries!