Upon the death of your horse you must return the passport to the issuing body within 30 days. They are required by law to invalidate the passport (and, if a microchip was implanted, the microchip number). You may however request that the issuing body return it once they have finished with it.
Death at an Abattoir
If your horse is slaughtered at an abattoir, it is the responsibility of the abattoir to return the passport to the issuing body within 30 working days. The horse can only be slaughtered if it is accompanied by a valid passport and Section II Part II shows it is intended for human consumption.