If a flying model (eg dragon) is facing an enemy, must he attack the front, or is he allowed to fly to the rear and charge?

Has to attack the front, all the basic rules apply.

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Coordinator of the Great British Hobbit League

Actually, he can fly over their heads and turn around in the movement phase, so that he is facing their rear, then charge their rear in the charge phase.

However, if you're talking about him facing their front once the movement phase is completed, then in the charge phase, you can only charge the arc that you're in (I.E. the front one).