There are many differences for UK folks. I am sure some of our UK friends will help out here.
I think it may be the parking brake issue as electric brakes are legal and allowed.
It is common on American caravans and trailers with electric brakes to have an electrical device mounted in the towing vehicle which the driver operates manually to apply the brakes of the trailer independent to the brakes on the tow vehicle. This is prohibited in Europe where the regulations demand that the service braking system must be applied without the driver removing his hands from the steering control.
Supply of electricity for electrical braking systems
The electrical energy required for the electrical braking system must be supplied to the trailer by the towing vehicle. If there is a battery on the trailer which is fed by the power supply unit of the towing vehicle, the power from the battery must be disconnected during application of the service braking system.
"American caravans and trailers do not always have a parking brake. European requirements demand that a caravan and trailer must be fitted with a parking brake that is capable of holding the vehicle stationary on an 18% up or down gradient. Also, the working parts must be held in the locked position by a purely mechanical device."
Ball size 50mm not 2' etc...