3D - the first standby would do the ejectee(s) - that is what it is theoretically there for and hence the RS15 - the seconds aircraft is for just about anything but was originally set up to continue the fast jet cover if the 1sts aircraft ended up away from base on a longranger or similar. Practically though, the seconds aircraft comes into its own for major incidents, providing a surge capability and the ability to deal with concurrent Sarops.
At the time of the July bombings (24/7), the RAF SARF generated 12 serviceable and ready to go aircraft and crews within 2 hours - fortunately they were not needed but that was 12 fully SAR capable crews just from the 6 RAF flights. Is anyone else going to provide that capability???????