Hello,
I remember reading something about a rash like yours. I found it while I was doing a ton of research to find out what is wrong with me-which I think is candida and/or parasites. It is called Dermatitis Herpetiformis and it usually occurs on the legs, arms, back, buttocks, and face or neck. It is part of celiac disease which is an intolerace to gluten in the intestines, common to british descendants. You can find a ton of information online.
I hope this helps you get one step closer.