Help required.

I have started to suffer from very serious prickly heat over the last few years. It is a extremelly ichy bright red rash all over my body and my arms become very puffy. This is so infuriatind as i love sitting in the sun and it turns up after being in ANY sun for over 1 day, I have tried all of the usual suggestions of, Cool showers, creams & ointments and Anti histamines(sorry about spelling)but none of this helps ease the iching
Can anyone reccomend a prevention method. (apart from staying out of the sun) a few pharmasists when i have been abroad have either said to get a cortizone injection or take iron tablets for 1 month prior to any sun but i dont know if these help.

Suggestions please