Please note that the law changes after 23/12/15 regarding horse passports.
ALL NEW PASSPORTS REQUIRE A COMPLETED SILHOUETTE AND MICROCHIP and vets can no longer issue chip-only passports.  New forms will be in use.
Only foals under 6 months can be issued a NEW passport all other passport applications will be stamped DUPLICATE or REPLACEMENT and stamped that the horse cannot enter the food chain.
If the passport is lost then a DUPLICATE must be applied for. If the animal cannot be identified a REPLACEMENT must be applied for.
Note - It is illegal to Sell a horse without a passport and it is illegal to transport a horse without a passport. The Buyer must register ownership within 30 days of purchase and if he/she wishes to sell on, their must br registered ownership prior to the sale. 
Finally, in the case of death the owner must return the passport to the issuing office with the date and cause of death within 30 days.  Fines of £5000 can be issued to owners failing to comply with any of the above.