From the links I posted earlier the form to use is the V112 which has sections specifically for miniatures, to quote from the link
"fill out the V112 as required (when asked to declare the category your machine falls into, if you do a lot of road runs, I would recommend – Category A for miniature steam tractors, traction engines, road locomotives and rollers. If you seldom venture out on the road you could put your engine in Category F if no more than 6 miles per week are being travelled. If your machine is a short wheel based Sentinel or Foden you could use one of the above two categories as well dependant on mileage, and for Sentinel or Foden wagons you could use Category D or again Category F if road use is seldom. If you own a STW electric powered Foden you would use Category J)."
Form V112 is used for MOT exempt vehicles.
They are also tax exempt
For those really curious the form can be downloaded from DVLA so you can see exactly what it says.
Edited By JasonB on 30/09/2016 10:59:05