Sorry i dont personally know but i used to work at a vet and spent a day trying to work this out for a cat going to South africa, slightly different BUT basically id try and ring your vets in england (ones that will be dealing with said horse) as they will be able to ring agencies/people who know. They will also hopefully know about the whole process and advise etc. I dont have the numbers at home and dont work there anymore otherwise i could find out... sorry.