Cost Saving
Surely regardless of ac type the biggest savings of all would be in manpower and hence in salary bills. The civilian SAR Flts have far fewer engineers and that is why they are cheaper. The ac don't fly all that many hours so surely the DOCs of the ac have a relatively lower impact. Could be argued that an old ac with little capital cost implications is a cheap way forward. Sea King 3s with the Carson blades could be looming onto the horizon!!
I agree with Crab that the Sea King is a good ac operationally. However, we should all remember that all the Flts whether RAF, RN or Civilian are all trying to do the best they can with the equipment they have. Equipment which inevitably was not designed from the outset for SAR but was compromised from some other role. None of them are perfect.