Hi,
I studied SDK100 a couple of years ago. The maths is honestly nothing to worry about. It's mainly scientific notations, power of 10's and percentages. There is so little of it though, and with it being a level one module all of the assessments (including the final one) are open book, so you can work out maths problems in slow time on paper with a youtube video in the background to explain the method to you if you'd like. If you want to do GCSE maths then please do, but it really isn't necessary in my opinion and could add extra unnecessary stress.
The topics are quite broad, and just sort of dip your toe into the specific area - they don't go into much detail, so very manageable. Some of the concepts in the module have been useful in my future study too, particularly the process of the action potential in neuronal communication which I think is found in the topic on pain (if it hasn't changed). One of my level 2 modules was SDK228, which I believe could also be on your psychology pathway, and understanding the biological basis of mood and affective disorders, plus the methods used to treat them and how they act on the brain was very helpful.
Always remember that your level 1 modules don't count to your final degree classification, you just have to pass them. So pick ones you find interesting and enjoy them!
Ben