Travel agents are less accountable than your staff, usually. the agents might follow your policy to the letter but that will almost always lead to either random higher costs that you didn’t expect, or unhappy staff when they get a less desirable itinerary,
And the corporate agents are the worst.
My own addition to the policy is a general state aim of not needing to travel outside of business hours. Not a hard and fast rule, but basically saying that employees have the choice of travel later, the night before or whatever if they choose to avoid the 0600 flight.
For me, I would prefer the 0600 to missing an extra bedtime for the kids, but someone else might not.
Also, generally, making sure your company recognises that no one really enjoys travelling outside of paid working time.