Dear Stuart ,
Banana Vans by definition were on restricted routeings like milk tanks , therefore even though they may be waybilled to say Birmingham , they would still have to be returned to a particular port such as Bristol or Southampton as that was where the cargo originated , and the wagons allocated at such ports would suffice for the traffic on offer.
Banana vans were steam heated , so even in wartime would most likely be restricted to food imports and not used for general traffic.
As Banana boats were specifically configured for bananas , regular loadings would be known in advance , so presumably a suitable number of wagons were allocated to accommodate that.
These days 70% of bananas for Britain come in via Portsmouth and are distributed by semi trailers.
As a generalization specialist non pool wagons were equipped for their specific traffic, and not suitable for general use , hence the reason for being non pool . Non pool wagons have always been allocated for specific services , hence specfic route brandings on some vehicles , and if private owner , being non pool would have a return destination either painted on the wagon or a special waybill with a specific return destination printed thereon.
Hope this helps,