Mystery "Cure" ?

I have had psoriasis for over thirty years and it affects my hands, lower arms, elbows, lower back, buttocks, knees and lower legs with occasional patches elsewhere. I use topical creams to keep it under control. I've tried most things over the years but now I just live with it. We are fortunate to have a house in southwest France and I have noticed that within a few days of arriving there my psoriasis improves, my hands clearing up almost immediately. This summer I spent almost three months there and my psoriasis has reduced to small patches just on my knees and elbows, all the other patches are completely clear. My diet contains more fresh and organic food (its France!) but is not substantially different from what I eat in England. I am fair skinned so I don't sunbathe and I keep covered up in the sun; yet patches on my back and buttocks have cleared up. In addition, my psoriasis improves during trips in the winter as well when its not particularly sunny. Interestingly, on returning to our house near London my psoriasis will almost immediately start to worsen; although fortunately, at a much slower rate. None of our many visitors has psoriasis so I have not been able to verify if this is unique to me. Can anyone offer any suggestions as to what is causing this mysterious 'cure'.?