Paul, a fellow in Ontario, Canada has been building 3-1/2" gauge box cars (North American) and these have resin wheels on 1/4" axles, and resin/plywood sides. They are as light as a feather. Some of the sides of refrigerated cars (with all of the colourful adverts they had over here) he got from a magazine called Model Railroader, and had them enlarged and glues the resulting scan to the side of the cars.
On close look they are missing detail, but they are fast to build, light to move, and colourful.
I believe the truck side frames are laser cut, with delrin bearings installed.
I know that it won't match a Britannia, (*) but the light-build techniques may suffice, and 20 wagons (say) may look better than 2 beautiful wagons when being pulled, not being inspected.
Good luck – lots of interesting discussions and new items for me to follow on this thread.
note * – one could always say that the Britannia was over on tour, like the Royal Scot, the Flying Scotsman, if one had the courage!
John.