They require the fitment of a breakaway switch that will operate the trailer brakes in the event of separation from the tow vehicle. They also require the fitting of a parking brake that is capable of holding the trailer when it is uncoupled from the tow. The standard 2 inch American hitch will also need changing.
But, "If buying used, is the trailer road legal and U.K. specification? Most American marine trailers are illegal even if the coupling and brakes have been converted to U.K. specification" From Witter's website.
Usually someone will come on here and say that in x number of years of using an American trailer they have never been challenged, but the problems start if you have an accident and your insurance company wont pay out.