Well, my local station, Princes Risborough, there aren't any barriers, but I frequently go to High Wycombe and Aylesbury, which both have barriers. I just buy the cheapest ticket available and go straight onto the platform to get the pics I want.
Generally, I only take pictures of other stations when I need to travel to them, but I've never been approached or questioned by anyone.
out of all my experience Chiltern are the worst TOC For Photos
I'm afraid all TOCs have their jobsworths

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I personally would just buy the cheapest return fare available, go onto the station and take the pictures. If anyone complains, just remind them of the National Rail guidelines that photography is allowed for personal use (which apply to ALL stations, regardless of TOC). If I want to take pictures, I generally avoid peak times if I can help it - I think taking pics in the peak times with all the busy trains around makes me look more 'suspicious' and thus the jobsworths will be even more keen to give you trouble :roll: