For those who keep questioning the lack of UK COD aircraft to carry large things like spare JSF aircraft engines, of course the Navy would like them, but we are where we are at the moment.
However, as mentioned by others, in the past when the Navy has not had such large COD aircraft to carry spare engines the Navy/MoD has always found ways to overcome that situation. Contingencies that allow for that have previously been put in place many times and if need be then surely, they could be implemented again!
For example, the MoD could adjust the required number of spares aircraft engines to be kept onboard a Carrier depending on the known reliability of such engines, the number of aircraft on the ship, the operational threat expected at that time, the maximum distance that the ship is likely to be from safe locations where spare engines could be obtained from, etc.
Note: Larger carriers have more space available to carry any additional engines, if required.
As others have also previously stated, RFA Supply ships can also carry whatever number of spare engines is required if there is a likely need for them.
When Carrier operational tasking is extremely urgent and ‘if’ spare engines are not available within the Naval task force, then replacement JSF aircraft could also possibly be flown out to the Carrier using Air to Air refuelling (if required) to get them there?
Why would the carrier always be expected to go close to shore to pick up any spare aircraft engines when another smaller ship could be sent to do that and then take the engine out to carrier’s location?
If the carrier was involved in a major war and was supporting NATO then there would also probably be a USA Carrier/Supplier ship in the same area and they may be able to offer the use/loan of a spare engine or provide a COD aircraft to collect the spare engine for our Carrier. Then its Osprey (or a helicopter) could transfer the engine to our carrier. (Maybe EH101 Merlins could also carry JSF Engines in transportation containers as underslung loads?)
If there is a need to extend the range of the Chinooks/Merlins then other ships could be positioned between the Carrier and the location where the Engine needs to be collected from (to allow extra refuel capability on route).
Note: If need be, some chinooks could also be upgraded to have air to air refuelling as mentioned below:
UK Looks At Helicopter In-flight Refueling
Finally, if we have not ordered enough spare JSF engines then it may be more cost effective to order a few more of those engines than to buy and set up a complete new squadron just for a few very expensive Ospreys. (Even if only as a short-term measure until we could hopefully one day afford to buy our own Navy Osprey aircraft.)