I don't think your father should be too concerned about it. If it has a CPH number that's fine, but your not duty bound to tell any purchaser the number. If the buyer doesn't know the number, they will have to apply for another one. If your concerned about potential purchasers using it for horses, therefore taking it out of being agricultural land, then that's something for them to look into and worry about after they buy the land - or for their solicitor to advise them about. No reason for your father to concern himself with it IMO.
As with most things, you can read up on all of the legislation and tie yourself in knots trying to work it out. In reality, most people would just start using the land and if at a later date they were advised they needed to apply for something then deal with it then.