Filling the dent is easiest, by some margin. I wouldn't be too worried about it, provided that the dent isn't deep enough that the back contacts the steerer (fork column).
Replacing the head tube would be about £150, plus a respray (Argos price list)
There is access to the rear of the dent, so it's not impossible that it could be pushed out by doing something like pulling or pushing through a section of round bar the same size as the inside of the head tube, with a long chamfer on one edge of it.