I think Chemistry would probably look better on your application, but if you think you can get better grades in Photography then go for it.
I only did 3 AS and A-levels, and still have been accepted into Russel Group universities, even after getting 2 A's and a D in my AS. Also, I am doing a BTEC in Applied Science, so I wouldn't worry about Photography being a seen as a 'soft subject'.
Also, your personal statement is almost as important as the grades you get!