Asthma and short-sightedness? I wish you well Typhoon, but I'd suggest you have a back-up plan in case RAF aircrew doesn't work out for you.
Don't mean to put a downer on your ambitions, but at 21 you're old enough not to have it sugar coated. The harsh fact is that either of those conditions on their own will likely preclude you from fast jet aircrew (and by extension, any other from of RAF/RN/AAC aircrew), and both together..., well.
I know this from experience.