Hi Roger,
You can easily get vehicle third liability insurance at the borders in the form of a border insurance policy. For example, in Morocco, directly after leaving the ferry at the port of Tangier, or at the border with Mauritania, etc. I have entered various insurance offices in the OB map here as POIs. Please take a look at the map. There you will also find many other POIs and information about the route.
Comprehensive insurance is relatively uncommon for travels like this and when available, usually comes with very high premiums. You would probably need to contact a broker who deals with special insurance. In Germany, I can recommend Tourinsure in Hamburg, which you can also find as a POI in the OB map. Alternatively, ask the insurer where the rig is insured in the UK, perhaps they can offer something for Morocco at least.
Have fun with the preparations and let me know if you have any further questions. Happy to help.
Bjoern