I cant comment on the humidity, cos its not something i pay any attention to.
However, i just wanted to say that i developed asthma very soon after moving to London... I dont know what caused it, or how i got it, but i basically got a terrible cough which lasted 3 months, so i finally took myself to the doctor about it and they told me that i'd developed asthma, and put me on inhalers. I've been on the inhalers ever since, even after leaving London.
Who knows? Perhaps i would have got it even if i had remained in Glasgow instead of coming to London?
Just wanted to let you know of my experience.