hi mate, i have been through (and still am going through)the asthma thing. i applied well over a year ago and have still not been offered a position. a lot of this is is due to an extremely poor third party occupational health adviser the frs used (they have since been binned to my delight)!
i declared use of an inhaler on my questionarre even though i had not used one for years. this set alarm bells ringing and they sent me for a histamine test. this determines the severity of asthma by the inhalation of verying doses of histamine. i presume a severe asthmatic would react quickly and therefore 'fail' the test. i failed the first time but because i had a cold, i was re-prescribed an inhaler (preventer) and re took the test a couple of months later, which i passed.
the chest specialist has since reported that my condition, with inhalers taken each day is suitable for using breathing apparatus ect. i now have to go for yet another medical with the new advisers but i have a specialists report, so there is still a tiny bit of light at the end of this very long tunnel!
other suggestions were an exercise test on a treadmill, and a gp letter (which said i was fine.)
i dont know whether or not i will be offered a post. just be prepared for the long haul because they take this VERY seriously, too seriously in fact. this is just one frs it may differ in others. if you are a moderate asthmatic whose symptoms are easily controlled with a preventer then there is a chance you may get in, but if you need a regular reliever or have allergic asthma, forget it.