To study Psychology..........Watch
If it's at A Level, it's just the same as any other essay based subject, in that you need to write coherently. I would say that if you were looking forward to degree level then writing is more important, but so long as you have a pass at GCSE you wouldn't be denied.
I would recommend at least a B in GCSE maths though. There is a lot of stats work in psychology (I have a degree in it) and although the stats are quite simple at A Level, you could still struggle if you don't have a firm grasp of statistics.