This is what I found.
Quote:
Part III
Track-Laying Vehicles(see application provisions below the following Table)
Table 1 2 3
Item No. Class of Vehicle Maximum Speed (in miles per hour) while vehicle is being driven on a road
1. A motor vehicle being a track-laying vehicle which is fitted with—(a) springs between its frame and its weight-carrying rollers, and(b) resilient material between the rims of its weight-carrying rollers and the surface of the road,and which is not drawing a trailer. 20mph
2. A vehicle specified in item 1 above drawing one or more trailers each one of which is either—(a) a track-laying vehicle fitted with springs and resilient material as mentioned in that item, or(b) not a track-laying vehicle and each wheel of which is fitted with either a pneumatic tyre or a resilient tyre. 20mph
3.A vehicle specified in item 1 above drawing one or more trailers any one of which is either—(a) a track-laying vehicle not fitted with springs and resilient material as mentioned in that item, or(b) not a track-laying vehicle and at least one wheel of which is not fitted with either a pneumatic tyre or a resilient tyre. 5mph
4. A motor vehicle being a track-laying vehicle which is not fitted with springs and resilient material as mentioned in item 1 above, whether drawing a trailer or not. 5mph
5. A motor vehicle not being a track-laying vehicle, which is drawing one or more trailers any one or more of which is a track-laying vehicle—(a) if every wheel of the motor vehicle and of any non-track-laying trailer is fitted with a pneumatic tyre or with a resilient tyre, and every trailer which is a track-laying vehicle is fitted with springs and resilient material as mentioned in item 1(b) in any other case 20
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The key being the phrase ' between the rims of its weight-carrying rollers and the surface of the road'.
I would argue that the wheel rim does not include the rubber tyre but a court may decide differently..... I would suggest a carrier is allowed to do 20 mph.