I will defer to serving colleagues but I think there is a difference between RN and RAF SAR. In the RN all helo crews are trained in SAR as they need it when deployed at sea. Their roles at Culdrose and Prestwick are primarily for civil rescue and hours paid for by the DTI - effectively an RN SAR tour is a respite (shore) tour from the front line, it is not a career path. In the RAF, SAR is there primarily for the rescue of downed RAF aircrew and is a career path in itself. In the RAF there is virtually no transfer into the front line for CSAR or vice versa, whereas in the RN any helo aircrew can ask for a SAR tour.
Re CSAR I thought Hoon announced a while back that the RN (via the JHC) were providing a nascent CSAR capability and the RAF Merlins would add to the role in due course.