Is it illegal to transport a horse box with horses in it if it breaks down on a motorway?

So you are asking if it is illegal to transport a box with horses that is already broken on a motorway ?

You can only transport those horses in a horse box that is fully secure and there is no way that its content would get away or break down during the travel.

Aroo?

How would you do that?

If you mean put the horse box on a wrecker or tow it, you wouldn't be able to do it safely for the horses. Presumably, it would tip the horse box making the horses fall.

People I know who trailer horses around have road plans that provide for service, or a rental box to transport the horses the rest of the way. The horses would have to be transferred into a working truck or trailer to continue on their journey, or the one they are in fixed on the spot.

You don't EVER want anyone with no experience to be pulling a trailer with horses in it. They may be excellent drivers, but of course transporting animals is a different issue entirely. I wouldn't allow it even if it were an option- and neither should you. Especially if they are not your animals.

Nope. As long as said failure in no way endangers the well being of the horses. Intentionally.