Your horse transporter should happily give you their Operators Licence number (its not a trade secret). Blue disc in the window is for UK mainland only. Green disc is international.
also you will need to check their SVS Licence for transporting-the welfare department in Worcester should be able to do this
Their lorry also should have the DEFRA Inspection Certificate for either under 8 hours, 8-12 hours UK only or International-there should be a certificate and a metal plate in the lorry
Also!!!! Check the transporters ACET or CET depending on whether its over 8 hours (ACET) or under 8 hours (CET)