You can indeed walk though from one side to the other without passing barriers; the barrier line is sideways on to the footbridge / passage at the platform entraces.
For photography, yes you do need a permit ( £40 for 30 days ) to use the station, but as one of the supervisors pointed out you do not need a permit to photograph from the nearby bridges; the one with the best view is about a 4 - 5 minute walk from the station.
As you come out up from the platforms, use the exit to your left ( past the toilets and down the stairsthen past a car park ); turn right at the end of the road and just keep walking, stay on the right hand pavement. It's the first bridge you come to.
Unfortunately it's a main road bridge so you can only cover one direction at a time, but it is an excellent viewpoint. Please excuse the poor quality of this picture; it was done with a rather unsatisfactory automatic compact while my SLR was in for repair recently.