Well said Bad Livin....
do the RN still allow some Rotary Wing Pilots to cross over to Fast Jet if they have the right aptitude
God yeah...big time...and not just Pilots..Observers as well. In particular the RN is looking to start training JSF pilots and are recriuting heavily for that aircraft.
With regards to the breathing problem...I think you'll find that the medical criteria for MCD are way way more stringent than for aircrew. If they won't allow you to be aircrew due to a wheeze, then there is no way they will allow you to be an MCD Officer.