On my recently completed Caravan Club towing course the instructor told us not to wrap the cable around the towball as it is unsafe practice as if by any chance the caravan comes off the hitch or isn't on correctly then there is nothing to hold the cable with so it obviously won't work.
He said best practice is to buy the stronger D clip & use that to clip the cable to the designated hole in the tow bar fixing, if you don't have one due to having a detachable towbar & no other hole to clip it to he did say you may have to wrap it around the tow ball but to bear in mind that it wouldn't really do the job correctly. He didn't say it was illegal just not a very good idea & to try to find secure fixing to it.
Luckily I have a hole in my towbar mount for the purpose so I clip it in there.