Mike, most Change of construction and use regulations have an inauguration date. Vehicle manufactured and/or registered before are exempt. Might it not help to post such dates?
As far as practical advice for towing a limber and gun. Depending on tow vehicle, the Morris' especially the early ones are a pain. The Mk1 has the accelerator pedal in the middle, the battery by your right eye, and zero peripheral vision. The later ones are some improvment, the CMP is a far nicer beast. The combination can be towed happily by a 101 or I've done it with a LWB Safari Land Rover. I have sen guns towed alone, but wouldn't be keen to try it myself. Better to put the gun on a twin axle bracked trailer.
Another problem is those behind getting to close and going under the gun barrel when braking. I've not yet found and answer to this, stupidty can't be cured. Th best defence is to make a muzzle cover with a large warning triangle attached, then add a red warning light in the middle.Seperate strong straps around the gun table to attach it to the carraige are also a must. The fitted locking handles aren't clever and the hooks can straighten
Due to the lousy side/rear visibility a second person out the top is advisable in tight corners.