Hi @ballkeeper there are a few available online....check out ebay for some examples.
Regulation 83 of the Road Vehicles (Construction and Use) Regulations 1986 concerns the towing of trailers by a wheeled vehicle - please see below:
As you can see from the above, item 2 of the table concerns ‘invalid carriages’ and no exceptions are listed. Therefore, invalid carriages can’t tow trailers.
The link below explains about the general use of class 3 invalid carriages:
We hope this information is of use.