Privately owned horseboxes are exempt from the directly applicable EU drivers’ hours rules if they do not exceed 7.5 tonnes and are being used for the non-commercial carriage of goods.
Generally speaking, the Vehicle and Operator Services Agency (VOSA) would normally consider that a vehicle not exceeding 7.5 tonnes is being used on a non-commercial basis where it is clear that the carriage of goods (in this case horses) is not with a view to making a profit. But ultimately, of course, it remains for the courts to interpret the law.