Excellent looking Osprey. What would the UK MoD designation be for this aircraft?
One could see something like an Osprey HM.1 thus following the HM=Helicopter, maritime as in the Merlin HM.1. That said, the tilt rotor part might cause angst and they already use the TR designation for Torpedo / Reconnaissance. Maybe VM for VTOL, Maritime thus giving an Osprey VM.1 designation as they don't appear to use the V designation.
Or, if rescue is being emphasized,
Osprey HAR.1 (Helicopter, Air Rescue). Or, as Greg suggests, go with a new 'V' prefix ... so,
Osprey VAR.1 (VTOL, Air Rescue)?