Lorries can escape the benefit-in-kind legislation as being neither a van nor a car and therefore the vehicle can be used with no personal tax consequences for the tax payer or their family.
However in order for the expenses of running the vehicle to be tax deductible the expense has to be for the purposes of the businesses. It is hard to see how most businesses require a horsebox!
If you are using your horse lorry to advertise your services by having your logo and business information painted on the lorry you may be able to argue a case for the expense being for the purpose of the business.
You should take advice from your accountant or call Clearways Accountants on 01737 244298 if you need further information on this subject.