I have cover from ERS through Sheawater. I already have cover for my car via my bank, so have the trailer assist. They can also do car and trailer, called Equicar, which covers your car when you aren't towing as well.
I've got cover through PRP. I had to use it once, about 4 years ago. We broke down 200 miles from home. They had us (horse and 3 humans) rescued in just over 2 hours and returned us home the same day. The car and trailer turned up the next day. Possibly the best £70 I have ever spent in horses.
Equine Rescue Services. I got car cover too, which I've used twice (thankfully never needed the trailer rescuing!). They're always super friendly and helpful. It's always surprising to me when they send somebody out to help you and you don't have to pay anything I expect the worst but the courtesy call is just that, checking everything went ok haha!