You don’t have to have a separate policy for a trailer as during use, when it is attached to a car that’s insured sufficiently, your trailer is covered by third party liability. This means if you cause any damage to property or other vehicles you will be insured. However this will not cover you for theft or damage to the trailer itself so it’s always advisable to take out a separate insurance to protect you in the event that it’s stolen or involved in an accident.