Observer training will be done by King Airs, though I think those Jet Streams are in service for a while yet.
The ex-Heron Flight Jet Streams are nearly worthless on the Civil market as they do not have civvy certification and IIRC the part numbers are different to civvy ac too so they cannot be broken and sold for spares. Only hope would be a foreign military, but with the whole world turning to King Airs I'd suggest that it's pretty unlikely.
The scuttle over their withdrawl is that the former CAS didn't take kindly to being ferried around in an ac that said "Royal Navy" on it and engineered the flight's demise....