Smegward
You can get the information from the AFCO or the RAF website. Below is an extract from the website under the heading "Medical conditions that preclude entry"
"If you suffer from asthma or have done in the past, you cannot be considered for flying branches of the RAF. For ground branches and trades, people with a past history of asthma, wheezing or inhaler use may be eligible for service following review by medical staff. If you have current asthma symptoms or a current prescription or you use an inhaler for asthma or wheeze (regardless of cause), you are not eligible to apply for service."
The first sentence make it quite clear. The key words are "or have done in the past". Basically, ANY diagnosis of Asthma at ANY age will stop you from flying. This policy became more strict (and clearer) only a few months ago. For ground branches, the 'four year prescription and symptom free' rule remains extant.
Link to the full list of issues which would preclude entry is here:
http://www.raf.mod.uk/careers/rafcms...A28D2D8DAE.pdf